Does Light Travel Forever?

Most recent answer: 01/23/2013

Hi Raja, Good question. First, let's think about why sound does not travel forever. Sound cannot travel through empty space; it is carried by vibrations in a material, or medium (like air, steel, water, wood, etc). As the particles in the medium vibrate, energy is lost to heat, viscous processes, and molecular motion. So, the sound wave gets smaller and smaller until it disappears. In contrast, light waves can travel through a vacuum, and do not require a medium. In empty space, the wave does not dissipate (grow smaller) no matter how far it travels, because the wave is not interacting with anything else. This is why light from distant stars can travel through space for billions of light-years and still reach us on earth. However, light can also travel within some materials, like glass and water. In this case, some light is absorbed and lost as heat, just like sound. So, underwater, or in our atmosphere, light will only travel some finite range (which is different depending on the properties of the material it travels through). There is one more aspect of wave travel to consider, which applies to both sound and light waves. As a wave travels from a source, it propagates outward in all directions. Therefore, it fills a space given approximately by the surface area of a sphere. This area increases by the square of the distance R from the source; since the wave fills up all this space, its intensity decreases by R squared. This effect just means that the light/sound source will appear dimmer if we are farther away from it, since we don't collect all the light it emits. For example, light from a distant star travels outward in a giant sphere. Only one tiny patch of this sphere of light actually hits our eyes, which is why stars don't blind us! David Schmid

(published on 01/23/2013)

Follow-Up #1: How far does light go?

Light just keeps going and going until it bumps into something.  Then it can either be reflected or absorbed.  Astronomers have detected some light that has been traveling for more that 12 billion years, close to the age of the universe.   

Light has some interesting properties.   It comes in lumps called photons.  These photons carry energy and momentum in specific amounts related to the color of the light.  There is much to learned about light.   I suggest you log in to our website and type  LIGHT into the search box.   Lots of interesting stuff there.

To answer your previous question "Can light go into a black hole?" ,  the answer is yes.

(published on 12/03/2015)

Follow-Up #2: less than one photon?

Certainly you can run the ouput of a single-photon source through a half-silvered mirror, and get a sort of half-ghost of the photon in two places. If you put ordinary photon detectors in those places, however, each will either detect zero or one. For each source photon, you'll get at most one of the detectors to find it. How does the half-ghost at the other one know whether it's detectably there or not? The name of that mystery is "quantum entanglement". At some level we don't really know the answer.

(published on 02/04/2016)

Follow-Up #3: stars too far away to see?

Most stars are too far for us to see them as individual stars even with our best telescopes. Still, we can get light from them, mixed with light from other stars. If our understanding of the universe is at all right, there are also stars that once were visible from here but now are outside our horizon so no light from them reaches us. It's probable that there are many more stars outside our horizon than inside, maybe infinitely more. It's hard to check, however, what's happening outside our horizon! It's even hard to define what we mean by "now" for things outside the horizon.

(published on 07/22/2016)

Follow-Up #4: light going out to space

Certainly ordinary light travels out to space. That's how spy cameras and such can take pictures of things here on the Earth's surface.

(published on 09/01/2016)

Follow-Up #5: end of the universe?

We don't think there's any "end" in the sense of some spatial boundary. Unless something changes drastically, there also won't be an end in time. The expansion looks like it will go on forever. So that wouldn't give a maximum range.

(published on 03/26/2017)

Follow-Up #6: seeing black holes

In principle a well-aimed beam would loop around the outside of the black hole and return to Earth. There aren't any black holes close enough to make this practical. Instead the bending of light by black holes is observed by their lensing effect on light coming from more distant objects.

The amazing gravitational wave signals observed from merging black holes provide even more direct and convincing proof that black holes exist and follow the laws of General Relativity.

(published on 01/29/2018)

Follow-up on this answer

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May 20, 2016

How does light travel?

by Matt Williams, Universe Today

How does light travel?

Ever since Democritus – a Greek philosopher who lived between the 5th and 4th century's BCE – argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas scientists ventured back and forth between the notion that light was a particle or a wave until the modern, the 20th century led to breakthroughs that showed that it behaves as both.

These included the discovery of the electron, the development of quantum theory, and Einstein's Theory of Relativity. However, there remains many fascinating and unanswered questions when it comes to light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle? And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?

Theory of Light in the 19th Century:

During the Scientific Revolution, scientists began moving away from Aristotelian scientific theories that had been seen as accepted canon for centuries. This included rejecting Aristotle's theory of light, which viewed it as being a disturbance in the air (one of his four "elements" that composed matter), and embracing the more mechanistic view that light was composed of indivisible atoms.

In many ways, this theory had been previewed by atomists of Classical Antiquity – such as Democritus and Lucretius – both of whom viewed light as a unit of matter given off by the sun. By the 17th century, several scientists emerged who accepted this view, stating that light was made up of discrete particles (or "corpuscles"). This included Pierre Gassendi, a contemporary of René Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton.

Newton's corpuscular theory was an elaboration of his view of reality as an interaction of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise "Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light". According to Newton, the principles of light could be summed as follows:

  • Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
  • These corpuscles are perfectly elastic, rigid, and weightless.

This represented a challenge to "wave theory", which had been advocated by 17th century Dutch astronomer Christiaan Huygens. . These theories were first communicated in 1678 to the Paris Academy of Sciences and were published in 1690 in his "Traité de la lumière" ("Treatise on Light"). In it, he argued a revised version of Descartes views, in which the speed of light is infinite and propagated by means of spherical waves emitted along the wave front.

Double-Slit Experiment:

By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction, interference and polarization of light, but was also because of various experiments that seemed to confirm the still-competing view that light behaved as a wave.

The most famous of these was arguably the Double-Slit Experiment, which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young's version of the experiment, he used a slip of paper with slits cut into it, and then pointed a light source at them to measure how light passed through it.

According to classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. Instead, the results showed that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen. This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide.

The only possible explanation for this pattern of interference was that the light beams were in fact behaving as waves. Thus, this experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, involving the discovery of the electron and electromagnetic radiation , would lead to scientists considering yet again that light behaved as a particle too, thus giving rise to wave-particle duality theory.

Electromagnetism and Special Relativity:

Prior to the 19th and 20th centuries, the speed of light had already been determined. The first recorded measurements were performed by Danish astronomer Ole Rømer, who demonstrated in 1676 using light measurements from Jupiter's moon Io to show that light travels at a finite speed (rather than instantaneously).

By the late 19th century , James Clerk Maxwell proposed that light was an electromagnetic wave, and devised several equations (known as Maxwell's equations) to describe how electric and magnetic fields are generated and altered by each other and by charges and currents. By conducting measurements of different types of radiation (magnetic fields, ultraviolet and infrared radiation), he was able to calculate the speed of light in a vacuum (represented as c).

In 1905, Albert Einstein published "On the Electrodynamics of Moving Bodies", in which he advanced one of his most famous theories and overturned centuries of accepted notions and orthodoxies. In his paper, he postulated that the speed of light was the same in all inertial reference frames, regardless of the motion of the light source or the position of the observer.

Exploring the consequences of this theory is what led him to propose his theory of Special Relativity, which reconciled Maxwell's equations for electricity and magnetism with the laws of mechanics, simplified the mathematical calculations, and accorded with the directly observed speed of light and accounted for the observed aberrations. It also demonstrated that the speed of light had relevance outside the context of light and electromagnetism.

For one, it introduced the idea that major changes occur when things move close the speed of light, including the time-space frame of a moving body appearing to slow down and contract in the direction of motion when measured in the frame of the observer. After centuries of increasingly precise measurements, the speed of light was determined to be 299,792,458 m/s in 1975.

How does light travel?

Einstein and the Photon:

In 1905, Einstein also helped to resolve a great deal of confusion surrounding the behavior of electromagnetic radiation when he proposed that electrons are emitted from atoms when they absorb energy from light. Known as the photoelectric effect, Einstein based his idea on Planck's earlier work with "black bodies" – materials that absorb electromagnetic energy instead of reflecting it (i.e. white bodies).

At the time, Einstein's photoelectric effect was attempt to explain the "black body problem", in which a black body emits electromagnetic radiation due to the object's heat. This was a persistent problem in the world of physics, arising from the discovery of the electron, which had only happened eight years previous (thanks to British physicists led by J.J. Thompson and experiments using cathode ray tubes).

At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein's explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle – a quantized form of light which he named "photons". For this discovery, Einstein was awarded the Nobel Prize in 1921.

Wave-Particle Duality:

Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg's "uncertainty principle" (which stated that measuring the position of a photon accurately would disturb measurements of it momentum and vice versa), and Schrödinger's paradox that claimed that all particles have a " wave function ".

In accordance with quantum mechanical explanation, Schrodinger proposed that all the information about a particle (in this case, a photon) is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly "collapse", or rather "decohere", to a sharply peaked function. This was illustrated in Schrödinger famous paradox involving a closed box, a cat, and a vial of poison (known as the "Schrödinger's Cat" paradox).

According to his theory, wave function also evolves according to a differential equation (aka. the Schrödinger equation). For particles with mass, this equation has solutions; but for particles with no mass, no solution existed. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.

When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits – tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more. As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.

The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson – a class of subatomic particles that are force carriers and have no mass.

So how does light travel? Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums (like air and water) as well as space. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly slow down or arrest the speed of light is gravity (i.e. a black hole).

What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the early 20th century, a revolution that we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we have learned much, but still have much to learn.

For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists look forward to. Someday, we just might have it all figured out!

Source: Universe Today

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  • Sound & Light (Physics): How are They Different?

How Does Light Travel?

Light bends at the interface of two media.

Sound & Light (Physics): How are They Different?

The question of how light travels through space is one of the perennial mysteries of physics. In modern explanations, it is a wave phenomenon that doesn't need a medium through which to propagate. According to quantum theory, it also behaves as a collection of particles under certain circumstances. For most macroscopic purposes, though, its behavior can be described by treating it as a wave and applying the principles of wave mechanics to describe its motion.

Electromagnetic Vibrations

In the mid 1800s, Scottish physicist James Clerk Maxwell established that light is a form of electromagnetic energy that travels in waves. The question of how it manages to do so in the absence of a medium is explained by the nature of electromagnetic vibrations. When a charged particle vibrates, it produces an electrical vibration that automatically induces a magnetic one -- physicists often visualize these vibrations occurring in perpendicular planes. The paired oscillations propagate outward from the source; no medium, except for the electromagnetic field that permeates the universe, is required to conduct them.

A Ray of Light

When an electromagnetic source generates light, the light travels outward as a series of concentric spheres spaced in accordance with the vibration of the source. Light always takes the shortest path between a source and destination. A line drawn from the source to the destination, perpendicular to the wave-fronts, is called a ray. Far from the source, spherical wave fronts degenerate into a series of parallel lines moving in the direction of the ray. Their spacing defines the wavelength of the light, and the number of such lines that pass a given point in a given unit of time defines the frequency.

The Speed of Light

The frequency with which a light source vibrates determines the frequency -- and wavelength -- of the resultant radiation. This directly affects the energy of the wave packet -- or burst of waves moving as a unit -- according to a relationship established by physicist Max Planck in the early 1900s. If the light is visible, the frequency of vibration determines color. The speed of light is unaffected by vibrational frequency, however. In a vacuum, it is always 299,792 kilometers per second (186, 282 miles per second), a value denoted by the letter "c." According to Einstein's Theory of Relativity, nothing in the universe travels faster than this.

Refraction and Rainbows

Light travels slower in a medium than it does in a vacuum, and the speed is proportional to the density of the medium. This speed variation causes light to bend at the interface of two media -- a phenomenon called refraction. The angle at which it bends depends on the densities of the two media and the wavelength of the incident light. When light incident on a transparent medium is composed of wave fronts of different wavelengths, each wave front bends at a different angle, and the result is a rainbow.

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About the Author

Chris Deziel holds a Bachelor's degree in physics and a Master's degree in Humanities, He has taught science, math and English at the university level, both in his native Canada and in Japan. He began writing online in 2010, offering information in scientific, cultural and practical topics. His writing covers science, math and home improvement and design, as well as religion and the oriental healing arts.

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Scientists develop transparent wood that is stronger and lighter than glass

Bob mcdonald's blog: a simple procedure results in see-through wood.

can light travel in wood

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Researchers at the University of Maryland have turned ordinary sheets of wood into transparent material that is nearly as clear as glass, but stronger and with better insulating properties. It could become an energy efficient building material in the future.

Wood is made of two basic ingredients: cellulose, which are tiny fibres, and lignin, which bonds those fibres together to give it strength.

Tear a paper towel in half and look closely along the edge. You will see the little cellulose fibres sticking up. Lignin is a glue-like material that bonds the fibres together, a little like the plastic resin in fibreglass or carbon fibre. The lignin also contains molecules called chromophores, which give the wood its brown colour and prevent light from passing through.

Early attempts to make transparent wood involved removing the lignin, but this involved hazardous chemicals, high temperatures and a lot of time, making the product expensive and somewhat brittle. The new technique is so cheap and easy it could literally be done in a backyard.

Starting with planks of wood a metre long and one millimetre thick, the scientists simply brushed on a solution of hydrogen peroxide using an ordinary paint brush. When left in the sun, or under a UV lamp for an hour or so, the peroxide bleached out the brown chromophores but left the lignin intact, so the wood turned white.

can light travel in wood

Next, they infused the wood with a tough transparent epoxy designed for marine use, which filled in the spaces and pores in the wood and then hardened. This made the white wood transparent.

You can see a similar effect by taking that same piece of paper towel, dip half of it in water and place it on a patterned surface. The white paper towel will become translucent with light passing through the water and cellulose fibres without being scattered by refraction.

The epoxy in the wood does an even better job, allowing 90 per cent of visible light to pass through. The result is a long piece of what looks like glass, with the strength and flexibility of wood.

can light travel in wood

As window material, it would be much more resistant to accidental breakage. The clear wood is lighter than glass, with better insulating properties, which is important because windows are a major source of heat loss in buildings. It also might take less energy to manufacture clear wood because there are no high temperatures involved.

Transparent wood could become an alternative to glass in energy efficient buildings, or perhaps coverings for solar panels in harsh environments. There could be no end of uses.

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Many different types of wood, from balsa to oak, can be made transparent, and it doesn't matter if it is cut along the grain or against it. If the transparent wood is made a little thicker, it would be strong enough to become part of the structure of a building, so there could be entire transparent wooden walls.

While this technology has yet to  be scaled up to industrial levels, the researchers say it has great potential as a new building material. In fact, they say that theoretically, an entire house could be made transparent. It is not clear why anyone would want to live in a transparent house, but for people who do, it would be OK to throw stones…

can light travel in wood

Images copyright Xia et al.  Creative Commons Attribution-NonCommercial license , 

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can light travel in wood

Bob McDonald is the host of CBC Radio's award-winning weekly science program, Quirks & Quarks. He is also a science commentator for CBC News Network and CBC TV's The National. He has received 12 honorary degrees and is an Officer of the Order of Canada.

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Incredible Answer

The Surprising Possibilities of See-Through Wood

Stronger than plastic and tougher than glass, the resin-filled material is being exploited for smartphone screens, insulated windows and more

Jude Coleman, Knowable Magazine

1-transparentw.jpg

Thirty years ago, a botanist in Germany had a simple wish: to see the inner workings of woody plants without dissecting them. By bleaching away the pigments in plant cells, Siegfried Fink managed to create transparent wood , and he published his technique in a niche wood technology journal. The 1992 paper remained the last word on see-through wood for more than a decade, until a researcher named Lars Berglund stumbled across it.

Berglund was inspired by Fink’s discovery, but not for botanical reasons. The materials scientist, who works at KTH Royal Institute of Technology in Sweden, specializes in polymer composites and was interested in creating a more robust alternative to transparent plastic. And he wasn’t the only one interested in wood’s virtues. Across the ocean, researchers at the University of Maryland were busy on a related goal: harnessing the strength of wood for nontraditional purposes.

Now, after years of experiments, the research of these groups is starting to bear fruit. Transparent wood could soon find uses in super-strong screens for smart phones; in soft, glowing light fixtures; and even as structural features, such as color-changing windows.

“I truly believe this material has a promising future,” says Qiliang Fu, a wood nanotechnologist at Nanjing Forestry University in China who worked in Berglund’s lab as a graduate student.

Wood is made up of countless little vertical channels, like a tight bundle of straws bound together with glue. These tube-shaped cells transport water and nutrients throughout a tree , and when the tree is harvested and the moisture evaporates, pockets of air are left behind. To create see-through wood, scientists first need to modify or get rid of the glue, called lignin, that holds the cell bundles together and provides trunks and branches with most of their earthy-brown hues. After bleaching lignin’s color away or otherwise removing it, a milky-white skeleton of hollow cells remains.

This skeleton is still opaque, because the cell walls bend light to a different degree than the air in the cell pockets does—a value called a refractive index. Filling the air pockets with a substance like epoxy resin that bends light to a similar degree to the cell walls renders the wood transparent.

The material the scientists worked with is thin—typically less than a millimeter to around a centimeter thick. But the cells create a sturdy honeycomb structure, and the tiny wood fibers are stronger than the best carbon fibers, says materials scientist Liangbing Hu, who leads the research group working on transparent wood at the University of Maryland, College Park. And with the resin added, transparent wood outperforms plastic and glass: In tests measuring how easily materials fracture or break under pressure, transparent wood came out around three times stronger than transparent plastics like Plexiglass and about ten times tougher than glass.

“The results are amazing, that a piece of wood can be as strong as glass,” says Hu, who highlighted the features of transparent wood in the 2023 Annual Review of Materials Research .

The Surprising Possibilities of See-Through Wood

The process also works with thicker wood, but the view through that substance is hazier, because it scatters more light. In their original studies from 2016, Hu and Berglund both found that millimeter-thin sheets of the resin-filled wood skeletons let through 80 to 90 percent of light. As the thickness gets closer to a centimeter, light transmittance drops: Berglund’s group reported that 3.7-millimeter-thick wood—roughly two pennies thick—transmitted only 40 percent of light.

The slim profile and strength of the material means it could be a great alternative to products made from thin, easily shattered cuts of plastic or glass, such as display screens. The French company Woodoo, for example, uses a similar lignin-removing process in its wood screens, but it leaves a bit of lignin to create a different color aesthetic. The company is tailoring its recyclable, touch-sensitive digital displays for products including car dashboards and advertising billboards.

But most research has centered on transparent wood as an architectural feature, with windows a particularly promising use, says Prodyut Dhar, a biochemical engineer at the Indian Institute of Technology Varanasi. Transparent wood is a far better insulator than glass, so it could help buildings retain heat or keep it out. Hu and colleagues have also used polyvinyl alcohol, or PVA—a polymer used in glue and food packaging—to infiltrate the wood skeletons, making transparent wood that conducts heat at a rate five times lower than that of glass, the team reported in 2019 in Advanced Functional Materials .

And researchers are coming up with other tweaks to increase wood’s ability to hold or release heat, which would be useful for energy-efficient buildings. Céline Montanari, a materials scientist at RISE Research Institutes of Sweden, and colleagues experimented with phase-change materials, which flip from storing to releasing heat when they change from solid to liquid, or vice-versa. By incorporating polyethylene glycol, for example, the scientists found that their wood could store heat when it was warm and release heat as it cooled, work they published in ACS Applied Materials and Interfaces in 2019.

The Surprising Possibilities of See-Through Wood

Transparent wood windows would therefore be stronger and aid in temperature control better than traditional glass, but the view through them would be hazy, more similar to frosted glass than a regular window. However, the haziness could be an advantage if users want diffuse light: Since thicker wood is strong, it could be a partially load-bearing light source, Berglund says, potentially acting as a ceiling that provides soft, ambient light to a room.

Hu and Berglund have continued to toy with ways to bestow new properties on transparent wood. Around five years ago, Berglund and colleagues at KTH and the Georgia Institute of Technology found they could mimic smart windows , which can switch from transparent to tinted to block visibility or the sun’s rays. The researchers sandwiched an electrochromic polymer—a substance that can change color with electricity—between layers of transparent wood coated with an electrode polymer to conduct electricity. This created a pane of wood that changes from clear to magenta when users run a small electrical current through it.

More recently, the two groups have shifted their attention to improving the sustainability of transparent wood production. For example, the resin used to fill the wood scaffolding is typically a petroleum-derived plastic product, so it’s better to avoid using it, Montanari says. As a replacement, she and colleagues invented a fully bio-based polymer, derived from citrus peels. The team first combined acrylic acid and limonene, a chemical extracted from lemon and orange rinds that’s found in essential oils. Then they impregnated delignified wood with it. Even with a fruity filling, the bio-based transparent wood maintained its mechanical and optical properties, withstanding around 30 megapascals of pressure more than regular wood and transmitting around 90 percent of light, the researchers reported in 2021 in Advanced Science .

Hu’s lab, meanwhile, that same year reported in Science Advances a greener lignin-bleaching method that leans on hydrogen peroxide and UV radiation, further reducing the energy demands of production. The team brushed wood slices ranging from about 0.5 to 3.5 millimeters in thickness with hydrogen peroxide, then left them in front of UV lamps to mimic the sun’s rays. The UV bleached away the pigment-containing parts of lignin but left the structural parts intact, thus helping to retain more strength in the wood.

The Surprising Possibilities of See-Through Wood

These more environmentally friendly approaches help limit the amount of toxic chemicals and fossil-based polymers used in production, but for now, glass still has lower end-of-life environmental impacts than transparent wood, according to an analysis by Dhar and colleagues in Science of the Total Environment . Embracing greener production schemes and scaling up manufacturing are two steps necessary to add transparent wood to mainstream markets, researchers say, but it will take time. However, they are confident it can be done and believe in its potential as a sustainable material.

“When you’re trying to achieve sustainability, you don’t only want to match the properties of fossil-based materials,” Montanari says. “As a scientist, I want to surpass this.”

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Light Year Calculator

What is light year, how to calculate light years.

With this light year calculator, we aim to help you calculate the distance that light can travel in a certain amount of time . You can also check out our speed of light calculator to understand more about this topic.

We have written this article to help you understand what a light year is and how to calculate a light year using the light year formula . We will also demonstrate some examples to help you understand the light year calculation.

A light year is a unit of measurement used in astronomy to describe the distance that light travels in one year . Since light travels at a speed of approximately 186,282 miles per second (299,792,458 meters per second), a light year is a significant distance — about 5.88 trillion miles (9.46 trillion km) . Please check out our distance calculator to understand more about this topic.

The concept of a light year is important for understanding the distances involved in space exploration. Since the universe is so vast, it's often difficult to conceptualize the distances involved in astronomical measurements. However, by using a light year as a unit of measurement, scientists and astronomers can more easily compare distances between objects in space.

As the light year is a unit of measure for the distance light can travel in a year , this concept can help us to calculate the distance that light can travel in a certain time period. Hence, let's have a look at the following example:

  • Source: Light
  • Speed of light: 299,792,458 m/s
  • Time traveled: 2 years

You can perform the calculation in three steps:

Determine the speed of light.

The speed of light is the fastest speed in the universe, and it is always a constant in a vacuum. Hence, the speed of light is 299,792,458 m/s , which is 9.46×10¹² km/year .

Compute the time that the light has traveled.

The subsequent stage involves determining the duration of time taken by the light to travel. Since we are interested in light years, we will be measuring the time in years.

To facilitate this calculation, you may use our time lapse calculator . In this specific scenario, the light has traveled for a duration of 2 years.

Calculate the distance that the light has traveled.

The final step is to calculate the total distance that the light has traveled within the time . You can calculate this answer using the speed of light formula:

distance = speed of light × time

Thus, the distance that the light can travel in 100 seconds is 9.46×10¹² km/year × 2 years = 1.892×10¹³ km

How do I calculate the distance that light travels?

You can calculate the distance light travels in three steps:

Determine the light speed .

Determine the time the light has traveled.

Apply the light year formula :

distance = light speed × time

How far light can travel in 1 second?

The light can travel 186,282 miles, or 299,792,458 meters, in 1 second . That means light can go around the Earth just over 7 times in 1 second.

Why is the concept of a light year important in astronomy?

The concept of a light year is important in astronomy because it helps scientists and astronomers more easily compare distances between objects in space and understand the vastness of the universe .

Can light years be used to measure time?

No , despite the name, you cannot use light years to measure time. They only measure distance .

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Light Seems to Pass through Solid Metal

can light travel in wood

Researchers directing a special type of light at metal poked with holes in irregular patterns recently discovered that all the light behaved like a liquid and fell across the metal to find its way through the escape holes.

That means the light was acting pretty weird. Picture shining a flashlight at your kitchen colander. While some of the light from the flashlight will travel through its holes, the solid part of the colander will keep much of the light from shining through.

In contrast, experiments described in the March 28 issue of the journal Nature demonstrated that terahertz radiation --a low-frequency light on the electromagnetic spectrum located between microwaves and mid-infrared regions--traveled around a thin sheet of metal, through patterned holes, and all of it came out the other side. Experts sometimes refer to this radiation as T-rays.

"You can get 100 percent transmission of light, even if holes only make up 20 percent of the area," University of Utah physicist Ajay Nahata told LiveScience . Nahata is one of the experimenters.

A 'surprising' earlier finding

Although it sounds simple, understanding how so much light can move around to fit through holes is a relatively new idea. An explanation started when Thomas Ebbesen illustrated in research published in 1998 that the amount of visible light that traveled through a single hole was more than scientists expected.

"It was surprising, because a hole is the simplest thing you could imagine," said electrical engineer Daniel Mittleman, who works in Rice University's T-ray lab but is not affiliated with the new study.

Since Ebbesen's findings, researchers have assumed that the theory only applied to light traveling through holes in periodic patterns, such as squares. But Nahata and physicist Z. Valy Vardeny found in the new experiments that light moved across the metal surface and passed through holes in a number of different irregular designs.

Nahata and Vardeny are also the first researchers to observe how terahertz radiation reacts with the metal and around the holes. While visible light oscillates so quickly that it's difficult to measure, scientists can accurately gauge the low frequency of terahertz radiation.

"By using terahertz, you can really see how and when light comes out of the holes," Mittleman told LiveScience . "Once you illuminate the hole, some light goes through and some comes out a little later."

T-rays and other light

Since all light waves tend to act similarly, Mittleman said, researchers can assume that the behavior they observe of the terahertz radiation also occurs across the electromagnetic spectrum.

The University of Utah researchers have high hopes for applications of terahertz radiation in wireless communication and homeland security operations.

Today, much of the low-frequency electromagnetic spectrum is crowded with communication and broadcasting signals. Terahertz is unchartered, promising territory, Nahata said, to open up more space for transmitting data at high speeds.

Also, since many everyday materials, such as clothing, plastics and wood look transparent under terahertz imaging , the technology could be used to spot concealed bombs and other explosive devices. In addition, materials absorb T-rays at varying frequencies, depending on the type of material. Anthrax, for example, can be detected with terahertz imaging by its frequency fingerprint.

"We're trying to make building block devices so we can go after a broad range of applications," said Nahata.

  • The Enduring Mystery of Light
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Can Light Travel Through Wood

Can light travel through wood?

Yes, light can travel through wood. However, the amount of light that travels through the wood depends on the type of wood and the thickness of the wood.

Oak and other hardwoods allow for more light to pass through than softwoods. And, the thicker the wood, the less light will pass through.

So, if you are looking to use wood as a light blocker, you may want to consider using a harder wood and a thicker piece of wood.

  • 1 Can light travel through a solid?
  • 2 What Cannot light travel through?
  • 3 Does light travel through aluminum foil or wood?
  • 4 Does wood absorb or reflect light?
  • 5 What materials can light go through?
  • 6 Can light travel through any material?
  • 7 Does light ever stop?

Can light travel through a solid?

Can light travel through a solid? This is a question that has puzzled scientists for centuries. In fact, it wasn’t until the early 1800s that a scientist named Thomas Young came up with an explanation.

According to Young, light is a type of energy that travels in waves. These waves can pass through certain materials, such as air and water, but they are slowed down when they pass through a solid. This is because solids are made of tiny particles called atoms, which are packed closely together.

The particles in solids are too close together for light to pass through them easily. This is why you can’t see through a solid wall, and why it takes a longer time for light to travel through a solid than through air or water.

What Cannot light travel through?

There are several things in the world that cannot allow light to travel through them. This is due to the way that light is affected by different materials. Some materials allow light to pass through them easily, while others do not allow light to pass through them at all.

One of the most common materials that does not allow light to travel through it is metal. Metals are great at reflecting light, which is why they are often used in mirrors. If light tries to travel through a metal, it will be reflected back out. This is why metal roofing is often used on houses, as it can keep the inside of the house cool by reflecting the sun’s heat away.

Another material that does not allow light to travel through it is glass. Glass is often used in windows and other forms of glassware because it allows light to pass through it nicely. However, if light tries to travel through a piece of glass that has been shattered, it will be reflected in all different directions. This is why it is important to never look at the sun through a piece of shattered glass, as it can cause serious damage to your eyes.

Does light travel through aluminum foil or wood?

Light is a type of energy that travels through the air and is used to see things. It is also used to power things like streetlights and headlights. There are different types of light, including visible light and ultraviolet light.

Visible light is the type of light that we can see. It is made up of different colors, including red, orange, yellow, green, blue, and purple. Ultraviolet (UV) light is a type of light that is invisible to us, but that can be seen by some animals.

There are different ways that light can travel. It can travel through the air, or it can travel through objects like water or glass. Light can also travel through materials like aluminum foil and wood.

Aluminum foil is a thin piece of metal that is made of aluminum. It is often used to wrap food, because it is a good conductor of heat. It can also be used to reflect light. When light hits aluminum foil, it is reflected off of the metal and away from the object that it is hitting.

Wood is a type of material that is made of trees. It is often used to build things, like houses and furniture. Wood can also be used to absorb light. When light hits wood, it is absorbed into the material and can’t be seen from the outside.

Does wood absorb or reflect light?

Wood is a natural material that is made up of small cells that are filled with sap. The cells in the sap are what determines how the wood will absorb or reflect light. In general, wood will absorb light, but the color and density of the wood will affect how much light is absorbed.

Darker woods will absorb more light than lighter woods. In addition, the grain of the wood will also affect how light is absorbed. Wood with a straight grain will reflect more light than wood with a curved grain.

There are also different types of wood finishes that can be used to either absorb or reflect light. A finish that is formulated to absorb light will make the wood look darker, while a finish that is formulated to reflect light will make the wood look lighter.

Overall, wood will absorb and reflect light depending on the type of wood, the color of the wood, and the type of finish that is used.

What materials can light go through?

This is a question that has been asked for centuries, and the answer is still not fully understood. However, scientists have been able to determine that some materials allow light to pass through them more easily than others.

The three main types of materials that light can pass through are transparent, translucent, and opaque. Transparent materials allow light to pass straight through them, without being scattered or reflected. This includes materials such as glass and air. Translucent materials allow some light to pass through them, but they also scatter and reflect light. This includes materials such as wax paper and thin plastic. Opaque materials do not allow any light to pass through them, and this includes materials such as metal and wood.

It is important to note that the term “light” can refer to different types of radiation. In the context of this article, light refers to visible light, which is the type of radiation that our eyes are able to detect. However, light can also refer to other types of radiation, such as ultraviolet radiation and infrared radiation. Different materials may be more or less effective at blocking these other types of radiation.

So, what materials are the best at allowing light to pass through them? In general, transparent materials are the best at letting light through, followed by translucent materials, and then opaque materials. This is why windows are often made of glass, and why milk is opaque but cream is translucent.

Can light travel through any material?

This is a question that has been asked for centuries, and the answer is not completely clear. Scientists have been able to determine that light can travel through some materials, such as air, water and glass, but it is not clear if light can travel through other materials, such as metal.

One of the main arguments against light being able to travel through metal is that metal is a conductor of electricity. This means that if light were to travel through metal, it would also travel along the electric currents in the metal. However, some scientists have argued that light can travel through metal, as light does not have the same properties as electricity.

Despite the ongoing debate, scientists have been able to develop ways to control the movement of light through metal. One way is to use a material known as an optical fiber. Optical fibers are made of glass or plastic, and they are able to guide light through them. This is done by using a technique called internal reflection.

Internal reflection occurs when light hits a surface and is then reflected back into the material. The angle at which the light hits the surface is important, as the angle needs to be greater than the critical angle. The critical angle is the angle at which the light is reflected so that it does not leave the material.

If the angle is less than the critical angle, the light will be refracted and it will leave the material. This is what happens when light passes from one material to another, such as from air to water.

By using optical fibers, scientists are able to control the direction that the light travels in. This is important, as it allows them to send the light to the specific place that they want it to go.

Despite the ongoing debate about whether light can travel through metal, scientists have been able to develop ways to control the movement of light through metal. This has allowed them to build devices, such as optical fibers, that can be used to direct the light.

Does light ever stop?

There is a lot of mystery and wonder around the concept of light. It’s one of those things that we see and experience every day, but often take for granted. We know that light is necessary for us to see, but what else is light capable of?

One question that people have asked for centuries is whether or not light ever stops. This is a difficult question to answer, as it depends on what you consider to be “light.” In general, light is understood to be a form of energy that is visible to the human eye. However, there are other types of radiation that are also considered to be light, such as microwaves, infrared radiation, and ultraviolet radiation.

So, does light ever stop? The answer is a bit complicated. In general, light will travel until it encounters an object or substance that absorbs or reflects it. For example, the sun emits light in all directions, and the Earth absorbs most of it. This is why we can’t see the sun in the day time. However, if there was a clear path to the sun, we would be able to see it.

Light can also be reflected by mirrors or other surfaces. For example, when you look in a mirror, you are seeing the light that is being reflected by the mirror. When you turn off the light in a room, the darkness is created by the absence of light.

So, in short, light does stop, but it’s not as simple as just saying that it does or doesn’t. It depends on the circumstances.

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What Makes Glass Transparent?

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can light travel in wood

Ever watch a house being built? Carpenters first erect the basic skeleton of the structure using two-by-four studs. Then they nail sheathing, usually plywood, to the studs to make walls. Most walls include a window opening, which holds a sheet of glass situated within a frame. Windows make a home feel bright, warm and welcoming because they let light enter. But why should a glass window be any more transparent than the wood that surrounds it? After all, both materials are solid, and both keep out rain , snow and wind. Yet wood is opaque and blocks light completely, while glass is transparent and lets sunshine stream through unimpeded.

You may have heard some people — even some science textbooks — try to explain this by saying that wood is a true solid and that glass is a highly viscous liquid. They then go on to argue that the atoms in glass are spread farther apart and that these gaps let light squeeze through. They may even point to the windows of centuries-old houses, which often look wavy and unevenly thick, as evidence that the windows have "flowed" over the years like the slow crawl of molasses on a cold day.

In reality, glass isn't a liquid at all. It's a special kind of solid known as an amorphous solid . This is a state of matter in which the atoms and molecules are locked into place, but instead of forming neat, orderly crystals, they arrange themselves randomly. As a result, glasses are mechanically rigid like solids, yet have the disordered arrangement of molecules like liquids. Amorphous solids form when a solid substance is melted at high temperatures and then cooled rapidly — a process known as quenching .

In many ways, glasses are like ceramics and have all of their properties: durability, strength and brittleness, high electrical and thermal resistance, and lack of chemical reactivity. Oxide glass, like the commercial glass you find in sheet and plate glass, containers and light bulbs, has another important property: It's transparent to a range of wavelengths known as visible light. To understand why, we must take a closer look at the atomic structure of glass and understand what happens when photons — the smallest particles of light — interact with that structure.

We'll do that next.

Electron to Photon: You Don't Excite Me

can light travel in wood

First, recall that electrons surround the nucleus of an atom , occupying different energy levels. To move from a lower to a higher energy level, an electron must gain energy. Oppositely, to move from a higher to a lower energy level, an electron must give up energy. In either case, the electron can only gain or release energy in discrete bundles.

Now let's consider a photon moving toward and interacting with a solid substance. One of three things can happen:

  • The substance absorbs the photon . This occurs when the photon gives up its energy to an electron located in the material. Armed with this extra energy, the electron is able to move to a higher energy level, while the photon disappears.
  • The substance reflects the photon . To do this, the photon gives up its energy to the material, but a photon of identical energy is emitted.
  • The substance allows the photon to pass through unchanged . Known as transmission, this happens because the photon doesn't interact with any electron and continues its journey until it interacts with another object.

Glass, of course, falls into this last category. Photons pass through the material because they don't have sufficient energy to excite a glass electron to a higher energy level. Physicists sometimes talk about this in terms of band theory , which says energy levels exist together in regions known as energy bands . In between these bands are regions, known as band gaps , where energy levels for electrons don't exist at all. Some materials have larger band gaps than others. Glass is one of those materials, which means its electrons require much more energy before they can skip from one energy band to another and back again. Photons of visible light — light with wavelengths of 400 to 700 nanometers, corresponding to the colors violet, indigo, blue, green, yellow, orange and red — simply don't have enough energy to cause this skipping. Consequently, photons of visible light travel through glass instead of being absorbed or reflected, making glass transparent.

At wavelengths smaller than visible light, photons begin to have enough energy to move glass electrons from one energy band to another. For example, ultraviolet light, which has a wavelength ranging from 10 to 400 nanometers, can't pass through most oxide glasses, such as the glass in a window pane. This makes a window, including the window in our hypothetical house under construction, as opaque to ultraviolet light as wood is to visible light.

Keep reading for more links that will illuminate your world.

Transparent Glass FAQ

Why is glass transparent to visible light but opaque to ultraviolet and infrared, why is glass transparent while any typical metal is opaque, is glass always see-through, how does sand become clear glass, why is glass transparent and brittle, lots more information, related articles.

  • Why is snow white?
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  • How Transparent Aluminum Armor Works

More Great Links

  • Sixty Symbols: Why is glass transparent?
  • Corning Museum of Glass
  • "amorphous solid." Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica, 2011. Web. (May 2, 2011) http://www.britannica.com/EBchecked/topic/21328/amorphous-solid
  • Askeland, Donald R. and Pradeep Prabhakar Phulé. The Science of Engineering and Materials. Thomson. 2006. Chandler, David L. "Explained: Bandgap." MIT News. July 23, 2010. (May 2, 2011) http://web.mit.edu/newsoffice/2010/explained-bandgap-0723.html
  • "glass." Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica, 2011. Web. (May 2, 2011) http://www.britannica.com/EBchecked/topic/234888/glass
  • Kunzig, Robert. "The Physics of … Glass." Discover Magazine. October 1999. (May 2, 2011) http://discovermagazine.com/1999/oct/physics/?searchterm=glass

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Structural changes in wood under artificial UV light irradiation determined by FTIR spectroscopy and color measurements – A brief review

UV weathering, a process initiated primarily by the ultraviolet portion of the solar spectrum, causes surface degradation of wood. Additionally, the wetting and drying of wood through precipitation, diurnal and seasonal changes in relative humidity, abrasion by air particulates, temperature changes, atmospheric pollution, oxygen, and human activities, all contribute to the degradation of wood surfaces. Photo-oxidation or photo-chemical degradation affects only the wood surface, starting immediately after exposure to sunlight. Understanding the chemistry of UV degradation of wood requires knowledge of the chemical nature of wood components, the UV spectrum, and the interactions of UV radiation with various chemical structures in wood. Chemical changes can be evidenced by FTIR spectroscopy. Previous study has shown that wood chemical modification with succinic anhydride makes it slightly more stable to the artificial light action than non-modified wood, which might be due to a slight increase in lignin stability to the polychromatic light action. Analysis of color changes on coated wood surfaces for modified wood treated with epoxidized soybean oil (ESO) has shown that lightness (Δ L *) decreases, whereas a *, b *, and Δ E * increase with increasing irradiation time.

Full Article

Structural Changes in Wood under Artificial UV Light Irradiation Determined by FTIR Spectroscopy and Color Measurements – A Brief Review

Carmen-Alice Teacă,* Dan Roşu, Ruxanda Bodîrlău, and Liliana Roşu

UV weathering, a process initiated primarily by the ultraviolet portion of the solar spectrum, causes surface degradation of wood. Additionally, the wetting and drying of wood through precipitation, diurnal and seasonal changes in relative humidity, abrasion by air particulates, temperature changes, atmospheric pollution, oxygen, and human activities, all contribute to the degradation of wood surfaces. Photo-oxidation or photo-chemical degradation affects only the wood surface, starting immediately after exposure to sunlight. Understanding the chemistry of UV degradation of wood requires knowledge of the chemical nature of wood components, the UV spectrum, and the interactions of UV radiation with various chemical structures in wood. Chemical changes can be evidenced by FTIR spectroscopy. Previous study has shown that wood chemical modification with succinic anhydride makes it slightly more stable to the artificial light action than non-modified wood, which might be due to a slight increase in lignin stability to the polychromatic light action. Analysis of color changes on coated wood surfaces for modified wood treated with epoxidized soybean oil (ESO) has shown that lightness (Δ L *) decreases, whereas  a *,  b *, and Δ E * increase with increasing irradiation time.

Keywords:   Wood; Chemical modification; Coating; UV irradiation; FT-IR spectroscopy; Color changes

Contact information: “Petru Poni” Institute of Macromolecular Chemistry, 41 A Gr. Ghica Voda Alley, Iasi, 700487, Romania;  * Corresponding author: [email protected], [email protected]

WOOD PHOTODEGRADATION PROCESS

Photo-degradation in wood caused by solar radiation is a complicated process. The intensity and wavelength of the solar radiation depends on a number of uncontrolled parameters. In addition, these parameters are changing not only throughout the year but even throughout the day. This is why artificial light sources are often used to clarify the mechanism of wood photo-degradation. Some of these light sources can imitate the contents of solar radiation, while others emit only a part (or some parts) of the spectra emitted by the sun. UV radiation represents only 4.6% of the solar spectrum, but it causes the most significant damage to polymeric materials. The complete solar UV spectrum ranges between 280 and 400 nm, but the most aggressive part is the UVB range with very short wavelengths between 280 and 315 nm (Fig. 1). Due to a need for more rapid evaluation of the resistance of materials to outdoor weathering, devices with artificial light sources are generally used to accelerate the degradation. These sources include filtered long arc xenon, fluorescent, metal halide lamps, and carbon arc lamps. Less commonly used sources include mercury vapor and tungsten lamps. These laboratory-accelerated weathering tests are more appropriately referred to as “artificial weathering”.

Fig. 1.  Components of the solar spectrum

The study of UV photo-degradation of wood began some decades ago (Kalnins 1966; Hon and Chang 1984). Such investigation typically has two steps: first irradiation of the wood’s surface by a UV-containing light beam and then detecting the changes caused by the radiation. Besides natural environmental weathering, several test methods have been developed using artificial light sources to provide accelerated test procedures. All methods are based on the regular observation of characteristics reflecting an ageing process such as mechanical properties and optical characteristics (crack formation, changes in color and gloss). Natural weathering tests provide the most accurate and reproducible data; however, their duration can be very long, which frequently instead favors the selection of an artificial weathering method.

Artificial weathering methods are useful tools for studying the durability of unfinished and finished wood products that will be used outdoors. Such weathering tests can accelerate the effects of natural weathering from 5 to 20 times depending on the exposure conditions chosen. The tests are valuable tools since the conditions can be controlled and reproduced. Because all the modes of natural weathering degradation cannot be simulated collectively (such as degradation by UV light, wetting by liquid water, and discoloration by mold and stain fungi), accelerated tests generally focus on the effects of UV light, moisture, and temperature.

Neither in a natural environment, nor with traditional UV lamp (xenon and mercury vapor lamps) experiments, is there a precise way to measure the energy of radiation on the wood sample’s surface. In addition, UV photo-degradation is influenced by other factors (moisture, temperature, visible light, and infrared radiation) that are difficult to exclude. Using UV-laser instead of traditional lamps during irradiation could solve all of the above-mentioned problems. By using lasers as radiation sources (Barta  et al.  2005; Papp  et al.  2005) the duration of the treatment can be shortened, the wavelength of the radiation is known, the energy can be determined, and the intensity of the radiation at the surface also can be determined.

Weathering chambers have been developed to provide a QUV® (registered trademark of Q-Lab Corporation, USA) weathering testing environment (ASTM G154). The QUV® simulates the effect of sunlight with fluorescent ultraviolet (UV) lamps, while rain and dew are simulated by the condensation of humid air. QUV® equipment employs mainly two types of lamps: UVA-340 and UVB-313 (where 340 and 313, respectively, represent the peak emission expressed in “nm”). These lamps have different light emission spectra, and are both characterized by maximum emission in the UV range.

Among all artificial UV sources, xenon lights provide the best simulation of natural sunlight. With an appropriate filter combination (usually a borosilicate type), the xenon light irradiance spectrum can be adapted to match closely to natural sunlight over a broad range of wavelengths.

Published studies have included in-air irradiation experiments of polymer materials, including wood samples with different chemical treatments, by means of a rotation device equipped with a middle pressure mercury lamp HQE-40 type, with 100W power, having a polychrome emission spectrum in the field of 240 to 570 nm (Fig. 2).

Fig. 2.  Emission spectrum of the applied mercury lamp

The more energetic types of radiation, with  λ  < 300 nm, are not found in the natural light spectrum, but they can be eliminated using a 30 μm borosilicate glass filter. A water filter and a fan are used to prevent the thermal degradation of the polymer sample during the photochemical treatment. The samples are mounted on the device, which is positioned at a distance of 60 mm from the lamp. The irradiance value and average radiant exposure measured on the sample surface are 97 Wm -2 , and 350 kJm -2 h -1 , respectively. The temperature inside the irradiation chamber is kept at around 40 to 45°C. The samples are withdrawn from the device after different irradiation times and further investigated for structural and color changes.

WOOD WEATHERING UNDER UV RADIATION ACTION

Wood Behavior under Exposure to Solar UV Radiation

Since ancient times, wood has been continuously present in the human environ-ment. Nowadays, in the context of the sustainability concept, the importance of wood as a renewable and ecological construction material is continuously increasing. Wood is used in many interior and exterior applications, for which an extended service life and conservation of both aesthetical appearance and mechanical properties are important. Under certain use conditions, however, wood is affected by biotic and non-biotic factors, causing degradation phenomena that will alter firstly the visual aspect and then the mechanical resistance of wood, hence limiting its service-life.

When wood is exposed outdoors above ground, a complex combination of chemical and mechanical factors contributes to what can be described as weathering, a long-term process resulting in a color change to grey, a roughened texture, and cracks following the combined action of ultraviolet radiation from sunlight and rainwater (Williams 2005). The same factors that cause weathering can actually facilitate the biological degradation of wood and diminish in time the efficacy of the protective treat-ments applied, namely wood preservation and coatings. Weathering should not be confused with decay, which results from decay organisms ( e.g ., fungi) acting in the presence of excess moisture and air for an extended period of time.

Not all fungi that attack wood cause degradation (Evans 2012). In fact, many are classified as wood-staining or mildew (mold) fungi because they discolor or stain wood rather than causing decay. These fungi typically develop because of poor drying practices or excessively wet conditions. Stain fungi do not cause strength loss but result in a lower grade and are considered unfavorable because of their appearance. Conditions that favor stain fungi are often ideal for wood-degrading organisms (Williams 2005).

Wood-staining fungi differ from the wood-destroying fungi in that wood-staining fungi do not noticeably affect wood strength or texture. Mold and stain are often considered together because of the similarity of action of the fungi on wood micro-structure. Mold and stain induce little injury to the structure of wood they infest. A number of wood-staining fungi produce a wide range of color effects or different stains. Control can be accomplished either by rapid drying of the wood to reduce moisture content or by dipping or spraying with fungicidal solutions. Dipping is usually fairly inexpensive. If the risk of stain is severe, both fungicidal protection and good drying practices are recommended for high-grade products.

In addition to discolorations produced by fungi, wood is also subject to certain other stains that result from chemical changes in wood cell walls. The nature and causes of these changes are not definitely known, although, in some cases at least, they involve enzymatic or non-enzymatic oxidation of certain organic compounds. Certain molds, similar to those that form on old bread, can form on the surface of wood and produce “cottony” growths that range from white and other light shades to black. These organisms differ from wood-staining fungi mainly in their habit of surface growth. The same conditions of moisture, air, and temperature that promote wood-destroying and wood-staining fungi will favor the growth of molds (Williams 2010).

Although molds (mildew) are more common with untreated wood, they can also be a problem in cases of preservative-treated wood. Annual treatment of the preservative-treated wood with a water-repellent preservative can reduce molds. Most unprotected wood will be discolored to a dull gray or black by mold. Preservative-treated wood that has not been treated with a water-repellent preservative will quickly turn to a dull gray or silver gray in some areas. Some molds are surprisingly tolerant of wood preservatives.

Because most molds need a surface moisture content of about 20 percent to begin growth, they can be controlled by controlling air moisture levels and minimizing condensation. If preventive measures fail, then other methods are available. The natural color of an outdoor wood structure can be partially maintained by scrubbing the wood surface annually with a bleach/water mixture or a commercial wood cleaner. The cleaned wood surface should be scrubbed with a stiff bristle brush and rinsed thoroughly with water. Always the wood surface needs to dry for several days before refinishing. However, aggressive cleaning methods to remove molds using strong chemicals and/or power-washing can greatly accelerate the loss of wood fiber from wood surfaces (Williams 2010).

The wood properties that vary greatly from species to species are: density, grain characteristics (presence of earlywood and latewood), texture (hardwood or softwood), extractives, resins, and oils content (Williams 2005). The wood density is one of the most important factors that affect weathering characteristics. This parameter varies to a large extent from species to species and it is important because hardwoods shrink and swell more than softwoods.

The amount of warping and checking that occurs as wood changes its dimensional stability under natural weathering process is directly related to wood density. High-density woods (generally hardwoods) tend to warp and check more than do the low-density woods (generally softwoods). Hardwoods are primarily composed of relatively short, small-diameter cells (fibers), and large-diameter pores (vessels); softwoods, in contrast, are composed of longer small-diameter cells (tracheids). The size and arrange-ment of the pores may enhance the effect of density and grain pattern on the weathering process. Hardwoods with large pores, such as oak and ash, may erode more quickly at the pores than the surrounding fibers.

Williams and co-authors reported on the effects of surface roughness, grain angle, exposure angle, and earlywood/latewood ratio on weathering rates of several softwood and hardwood species (Williams  et al.  2001 a, b, and c). Erosion was slower for most hardwood species and faster for low-density softwood ones. For all wood species, the erosion rates for earlywood and latewood were greatly different during the first 7 years of weathering (outdoor experiments spanned 16 years). The erosion rates of all species were considerably higher at 45° exposure angle than at 90°.

Latewood is denser, harder, smoother, and darker than earlywood, and its cells have thicker walls and smaller cavities. The wider the latewood band, the denser will be the wood. Differences in morphology have been shown to affect weathering of the wood surface. Wood species with wide latewood bands weathers differently than do species with thin latewood bands. As wood weathers, the extractives are leached from the surface, which becomes less water-repellent. The loss of lignin also makes the surface more hydrophilic. Contact angle measurements on weathered western red cedar dropped from 77˚ to 55˚ after four weeks of outdoor weathering (Kalnins and Feist 1993).

In addition to the slow erosion of the wood surface under weathering conditions, the surface also can develop checks and raised grain. This type of degradation is often more severe than erosion. For example, wooden decks are often replaced long before their expected service life because of raised grain and splitting. This degradation is caused primarily by moisture. During outdoor weathering, water mechanically abrades the wood surface and removes degradation products. In addition, water hydrolyzes mainly hemicelluloses at the wood surface, noting that these become more susceptible to hydrolysis as lignin degrades.

Studies of wood-weathering phenomena are numerous, and principle breakdown processes during exposures are generally known; however, results obtained using different methods are incoherent and dependent on the particularities of the applied testing method. Weathering occurs through the combined effects of UV-light, water, oxygen, heat, and atmospheric pollutants such as SO 2 , NO 2 , and O 3 . Wood surfaces become rough as the grain raises during the weathering process. The dimensional stability of the wood is also affected as checking and cracking become severe.

The roughened surface changes in color and becomes more susceptible to decay and may fragment (Feist and Hon 1984). Ultraviolet radiation from sunlight is the most damaging component of the outdoor environment because it initiates a wide variety of chemical changes in wood surfaces, which lead to discoloration and deterioration. The surface of wood becomes grey, rough, and stringy. It loses not only its usual appearance, but also its mechanical properties, as the strength characteristics degrade. These damages caused from exposure are called degradation processes.

Wood is capable of absorbing several different wavelengths of electromagnetic radiation, initiating photo-chemical reactions leading to wood discoloration. Wood contains cellulose, hemicellulose, lignin, and extractives. Wood polymer components contain internal chemical labile entities such as carbonyl, carboxyl, aldehyde, phenolic hydroxyl, unsaturated double bonds, and external entities such as waxes, fats, and metal ions. All wood polymers are sensitive to ultraviolet radiation (Fig. 3). Solar radiation depolymerizes lignin and cellulose, and water leaches the resulting photo-degraded fragments from the wood (Derbyshire and Miller 1981; Evans  et al.  1993).

Fig. 3.  Main chemical constituents of plant and wood biomass

Natural biopolymer materials undergo UV-induced discoloration, usually resulting in an increase in the yellowness on exposure. Lignocellulosic materials such as wood and paper readily undergo light-induced yellowing (Hon 1991).

A schematic representation of light action upon wood biomass is presented in Fig. 4. While both the cellulose and lignin constituents are sensitive to solar radiation, it is the latter that is mostly responsible for the phenomenon. Lignin, which makes up 29 to 33 wt% of softwood, contains numerous chromophores that efficiently absorb UV radiation. Lignin is responsible for absorbing 80 to 95% of the total UV light absorbed by wood, carbohydrates 5 to 20%, and extractives about 2% (Norrstrom 1969).

Although the weathering of wood materials depends on many environmental factors, only a relatively narrow band of the electromagnetic spectrum, the UV-light portion of sunlight, is responsible for the primary photo-oxidative degradation of wood. The first law of photochemistry, the Grotthus–Draper principle, states that for a photo-chemical reaction to occur, some component of the system must first absorb light. The second law of photochemistry, the Stark–Einstein principle, states that a molecule can only absorb one quantum of radiation at a time (Rabek 1995). The absorbed energy can cause the dissociation of bonds in the molecules of the wood constituents. This homolytic process produces free radicals as the primary photochemical products. This event, with or without the participation of oxygen and water, can lead to depolymerization and to the formation of chromophoric groups such as carbonyls, carboxyls, quinones, peroxides, hydroperoxides, and conjugated double bonds (Feist and Hon 1984).

Fig. 4.  Schematic representation of light action upon wood biomass

Most of the components in wood are obviously capable of absorbing enough visible and UV light to undergo photochemical reactions leading ultimately to discoloration and degradation. Because of the wide range of chromophoric groups associated with its surface components, wood cannot easily be penetrated by light. Essentially, discoloration of wood by light is a superficial surface phenomenon.

Apart from a decrease in the methoxyl and lignin contents, and an increase in the carboxyl content inside the wood (Leary 1967; Leary 1968), photo-degradation also results in an increase in cellulose content and a decrease in lignin content on the wood surface (Wang and Lin 1991; Evans  et al.  1992; Hon 1994, 2001; George  et al.  2005). This result ultimately leads to a reduction of some physical, chemical, and biological properties of natural wood.

The discoloration of wood is attributed to the modification of the chromophores, which are capable of absorbing UV light in the range of 300 to 400 nm. Another indication of chemical change on the wood surface is the decrease in lignin content relative to cellulose content on the weathered wood surfaces. There have been many studies attempted to clarify the mechanism of wood weathering (Hon 1983; Hon and Chang 1984; Pandey 2005a; George  et al. 2005; Evans 2012). It has been shown that the degradation process is triggered by the formation of free radicals by UV irradiation (Moore and Owen 2001; Müller  et al.  2003).

The wood constituent polymers show different capacities with respect to absorbing UV light to form radicals. Lignin is extremely susceptible to UV irradiation, leading to the formation of aromatic free radicals (phenoxyl radicals), which further react with oxygen to produce carbonyl and carboxyl groups (Pandey 2005), these being related to wood discoloration by the occurrence of unsaturated carbonyl compounds (quinones). The presence of lignin in the formulation of wood-plastic composites (WPCs) can accelerate the composites’ photo-degradation under UV weathering (Chaochanchaikul  et al. 2012).

Photo-degradation of WPCs involves several factors including the wood flour content, coupling agents, manufacturing methods, and weathering conditions, among others. Matuana and Kamdem (2002) investigated the influence of accelerated artificial weathering on the color changes and tensile properties of PVC/wood-flour composites with various wood flour contents. While the composites retained their original tensile properties, they experienced severe discoloration (darkening) after weathering for 2600 h. PVC/wood-flour composites containing 15% wood flour were darkened more than those containing 30% and 45%   wood flour. Conversely, reports indicate the opposite trend for weathered HDPE/wood-flour composites; the composites containing 50% wood flour were lightened more than their counterparts with 25% wood flour (Stark and Mueller 2008). Ndiaye and coworkers (2008) studied the effects of wood flour content and compatibilizer on the durability of HDPE-based and PP-based WPCs. Their results indicated that the oxidation rate of the composites increases with wood content and decreases with the presence of compatibilizer due to better dispersion of wood flour in polymer matrix.

Some investigations have studied the effects of natural and accelerated artificial weathering on the appearance and chemical changes of HDPE-based and PP-based WPCs (Muasher and Sain 2006; Stark and Matuana 2004a; Stark and Matuana 2004b; Matuana and Kamdem 2002; Stark and Mueller 2008; Matuana  et al.  2001; Stark and Matuana 2007; Stark 2006; Stark  et al.  2004; Stark and Matuana 2006; Fabiyi  et al.  2008). The lightness, total color change, and wood loss increased with exposure time. PP-based WPCs experienced quicker photo-degradation in terms of lightness and wood loss. Delig-nification and oxidation accounted for the increased lightness of weathered composites. Muasher and Sain (2006) also investigated the discoloration of HDPE-based WPCs exposed to natural weathering and reported that WPCs undergoes two competing redox reactions upon UV exposure: the formation of paraquinone chromophoric structures generated by the oxidation of lignin, resulting in yellowing, as well as the reduction of the paraquinone structures to hydroquinones, leading to photo-bleaching. The yellowing mechanism dominant during the first 250 hours of exposure precedes the photo-bleaching mechanism, which becomes dominant with increased exposure time.

Evidence of Chemical Changes in Wood after Exposure to Artificial UV Radiation

Wood is a good absorber of infrared (IR), visible, and ultraviolet (UV) light. UV light is fully absorbed by a 75 μm-thick layer of wood (Hon and Ifju 1978). The penetra-tion depth profile of light into wood, which was found to be dependent on the wavelength of the light, was investigated by Kataoka  et al.  (2004; 2005; 2007). The effects of UV light irradiation can be detected at a depth of 400 to 500 μm for aromatic skeletal vibration (1510 cm -1 ) and at a depth of 600 to 700 μm for carbonyl vibration (1730 cm -1 ) under long-term treatment (Kataoka and Kiguchi 2001). This means that wood is not transparent for light, and the usual transmission method is not suitable for measuring the absorption properties of wood. For this purpose, powdered wood is mixed with a light-transparent material (usually and widely used potassium bromide). This mixture is pressed into pellets, and the pellets are used to measure the wood absorption using the transmission method. This method constitutes an excellent analytical tool to reveal evidence of the chemical composition of wood by IR spectroscopy. Since the change caused by irradiation appears in a thin surface layer, it is not easy to remove the proper thin layer for pellet preparation. In the case of wood, it is difficult to remove a layer thinner than 80 μm that has suffered change during the treatment. Concerning this difficulty, many investigators have used the pellet method without indicating the thick-ness of the wood layer (Hon and Feist 1986; Košiková and Tolvaj 1998; Müller  et al.  2003; Pandey 2005; Umemura  et al . 2008).

The Fourier transform IR technique can measure the diffusely reflected (DRIFT) intensity of a thin layer. This is a useful technique to investigate photo-degradation because the information is given for the same layer on which the photo-degradation process occurred. The absorption properties of the surface layer using the reflected light can be calculated by means of the Kubelka–Munk (K-M) theory (Kubelka and Munk 1931; Kubelka 1948). This theory was created for poorly absorbent materials. Since photo-degradation is a surface phenomenon and wood is a good light absorber, the K-M theory is widely applied to determine the light absorption of wood (Owen and Thomas 1989; Horn  et al.  1994; Tolvaj and Faix 1995; Zanuttini  et al.  1998; Košıková and Tolvaj 1998; Barta  et al.  1998; Pandey and Theagarajan 1997; Pandey 1999; Moore and Owen 2001; Weiland and Guyonnet 2003; Mitsui  et al.  2003; Cui  et al.  2004; Papp  et al.  2004; Kishino and Nakano 2004; Tolvaj and Mitsui 2004; Tolvaj and Mitsui 2005). In order to obtain reproducible FTIR spectra from wood surfaces, the roughness and the anatomical directions of the cut sections must be constant. It is difficult to compare the FTIR spectra of different solid wood surfaces because the natural roughness of the wood species varies widely. The validity of the Kubelka–Munk (K-M) theory was investigated to determine the IR absorption spectra of wood based on DRIFT measurements taken on photo-degraded samples (Tolvaj  et al. 2011). It was evidenced that the measured K-M function can be used as an absorption spectrum if its values are below 14 K-M units. Above this limit, the K-M theory, which was created for poorly absorbing materials, does not give the absorption of wood properly. If a matt aluminium plate is used as a background material and the values are between 14 and 40, absorption changes can be calculated after normalization of the spectra. This manipulation is only successful if there is an absorption peak close to the examined one which does not change its absorption during the photo-degradation process.

Infrared studies revealed that, during UV irradiation of wood, absorption due to carbonyl groups at 1720 and 1735 cm -1  increased, whereas the absorption for lignin at 1265 and 1510 cm -1  gradually decreased. The increment of carbonyl groups was the result of oxidation of cellulose and lignin, and the reduction in the amount of lignin was due to its degradation by light (Feist and Hon 1984). A convenient measure of the change in carbonyl groups and lignin is given by the ratio of the IR absorbance bands of carbonyl groups and lignin to the absorption band at 895 cm -l , an absorption band due to hydrogen located at the C1 position, which is normally unchanged during photo-irradiation.

The aesthetic appearance of wood is an important factor concerning wood quality, and color and has a significant impact on each individual’s perception of this appearance. Unfortunately, wood surfaces undergo drastic color changes when exposed outdoors, and they mostly turn grey. In order to compare the quality of different wood products, it is necessary to determine the rate of discoloration that wood surfaces undergo when exposed outdoors. Color can be measured using a wide range of spectrophotometers according to the EN ISO 11664-4 (Colorimetry – Part 4: CIE 1976  L * a * b * Color space). The CIE L * a * b *  system is one of the systems used to quantify color. The color parameters of this system are as follows: the  L*  axis is the lightness (ranging from 0 (black) to 100 (white)) and  a*  and  b*  axes represent the chromaticity coordinates (a positive  a*  value refers to red and a negative  a*  value to green while  +b*  and  –b*  denote yellow and blue, respectively). The color difference  E*  is the combined effect of the parameters and can be calculated according to Equation 1:

Δ E ab *  = [(Δ L * ) 2   + (Δ a * ) 2   + (Δ b * ) 2 ]  1/2   (1)

Generally, wood is colored by painting. Wood painting has mainly two object-tives: one to change the color, and the other to protect the surface. The emission of volatile organic compounds such as toluene and xylene from paint, however, is a health concern. Coloring by irradiation and heat treatment emits no volatile organic compounds and is very simple (Mitsui  et al.  2001). Furthermore, there is less damage, such as cracking and bending, because the wood is treated at a comparatively low temperature. The change in the lightness, Δ L *, of light-irradiated wood with heat treatment (both dry heating at 120°C, 140°C, and 160°C in air and humid heating at 50°C, 70°C, and 90°C, at a high relative humidity 90% RH for up to 150 h) was much greater than that of un-irradiated wood. With low temperature treatment, the color of irradiated wood (Δ a * and Δ b * parameters) changed remarkably with high relative humidity; therefore, heat and the presence of water accelerated the change in the color of irradiated wood. It is thought that the amount of change in color is due to the light-irradiation time and heat treatment conditions ( i.e ., heating temperature, time, and relative humidity). Moreover, it is reported that the ultraviolet region is effective in this method. This treatment can be a useful alternative as a new coloring method.

CHEMICAL TREATMENTS AGAINST WEATHERING PROCESS

Wood Coating Treatments

Wood is a material sensitive to humidity because of the hydrophilic nature of its cell wall constituent polymers cellulose, hemicelluloses, and lignin. Moist wood is vulnerable to attack by fungi and termites, these affecting its dimensional stability. The polar surface associated to the hydroxyl nature enables the establishment of strong hydrogen bonding between wood fibers, as a three-dimensional network.

To obtain long-term service life, wood must have proper protection. There are several approaches to improve water-repellency and dimensional stability of wood, including immersion-diffusion or vacuum-impregnation with preservatives, heating, brushing paint, and surface coating (Forsthuber and Grüll 2010; George  et al.  2005; Mitsui  et al . 2003; Mitsui  et al.  2001). One emerging technique is chemical modification, where chemicals such as anhydrides, isocyanates, alkyl chlorides,  etc ., react with hydroxyl groups,  i.e ., with the most reactive groups of cell wall polymers (Rowell 2005).

For economic reasons and for simplicity, surface coating has long been preferred to modifications by chemical immersion. The primary function of any coating is to prevent moisture penetration, improve resistance to weathering, and maintain the natural appearance of wood. A common method to achieve coating and wood extensive protection is the addition of pigment particles to a coating. These particles absorb, reflect, or scatter the incoming UV radiation and hinder it from reaching the wood substrate. The most effective UV absorber known is carbon black, but also other pigments such as TiO 2 are in common use today. Another effective treatment that protects against photo-chemical degradation and is water repellent is treatment of wood with chromic acid. This compound alters the molecular structure of lignin, and is remarkably effective at photo-stabilizing wood (Schmalzl  et al.  2003). Due to the molecular structure of this chemical it is classified as especially hazardous, being toxic, corrosive, and carcinogenic. Attempts to find less harmful metal compounds to contribute at photo-stabilizing wood surfaces have been undertaken without effective results (Schmalzl and Evans 2003).

However, in many cases other types of photo-stabilizers are required to protect the wood, without coloring the surface. One of the most common ways of achieving this is to add low molecular weight substances that have the ability to absorb some of the incoming radiation from the sun and then dissipate it as heat. Such substances are called UV absorbers (UVA) and can be used to protect both the substrate and the coating itself. Examples of such substances are benzotriazole, triazine (Hayoz  et al.  2003), and also substituted 2-hydroxybenzophenones which, due to their structure, can convert UV energy into thermal energy by intramolecular hydrogen transfer or  cis – trans  isomerization (Evans and Chowdhury 2010).

Other types of substances that are commonly used for UV protection in coatings are the hindered amine light stabilizers (HALS). These are amines with two methyl groups; such compounds undergo photo-oxidative conversion, thus hindering degradation of the coating and the substrate (George  et al. 2005). Use of UV absorbers (UVA) and hindered amine light stabilizers (HALS) both in the coating system and also in a direct wood impregnation step can improve the light stability considerably (Schaller and Rogez 2007; Forsthuber and Grüll 2010). Usually, an efficient protection strategy is to impreg-nate the wood surface with a solution of HALS, which will inhibit or at least reduce the lignin sensitivity to photo-oxidation, before applying the transparent top coat containing the UV absorber.

An alternative route to increase the hardness, dimensional stability, decay and acid resistance of wood is to impregnate wood with low molecular weight phenol formaldehyde (PF) resin, which penetrates the wood cell walls and can be cross-linked by heating (Furuno  et al.  2004). PF resins are good absorbers of UV light. The resistance of wood treated with low molecular weight PF resin to weathering can be improved by increasing the concentration of PF resin and by combining it with a water-soluble hindered amine light stabilizer HALS (Evans  et al.  2012). This approach may be justified in some higher value exterior applications of wood.

In recent years, the development of inorganic coatings on polymer surfaces has been fostered because of their excellent mechanical and thermal performance, optical properties, and bacterial resistance. On the other hand, the presence of the hydroxyl groups can promote the nucleation and growth of inorganic phases such as TiO 2  and SiO 2  (Schmalzl and Evans 2003; Tshabalala and Gangstad 2003; Barata  et al.  2005; Kim et al. 2006; Tshabalala and Sung 2007; Mahltig  et al.  2008; Kuroda  et al.  2008; Chen  et al.  2009; Li  et al.  2010; Sun  et al.  2011) on the wood surface, thus producing organic–inorganic hybrid materials. These inorganic particles are usually generated from their monomer precursors containing reactive organic groups to build up chemical bonds to the hydroxyl groups of wood. Encountering the hydroxyl groups of wood, the monomer precursors would be hydrolyzed, generating inorganic particles and reactive organic groups with the continuous condensation reactions to form a solid inorganic coating on the wood surface. These inorganic particles have great potential as photo-protective agents for wood because of their high chemical stability, nontoxic nature, and ability to absorb or scatter UV irradiation (Hon 2001; Ncube and Meincken 2010).

The aesthetic value of wood can quickly be lost if left unprotected from weathering. The absorption of UV radiation in the coating and the underlying wood substrate can cause a series of complex chemical reactions that amongst other effects, resulting in the loss of adhesion between the coating and the substrate. Degradation can occur either in the coating, on the wood surface, or in both materials (Chang and Chou 1999). Wood materials with significantly improved UV resistance were successfully fabricated by growing highly ordered ZnO nanorod arrays on wood surfaces using a facile one-pot hydrothermal method (Sun  et al.  2012).

Inorganic nanoparticles, for example CeO 2  and ZnO, have the same function as pigment particles, although they are much smaller in size and are hence transparent in the visible light spectrum. CeO 2  and ZnO have significant absorption in the UV region of the light spectrum, indicating that these substances could be interesting for use as UV absorbers in coatings (Blanchard and Blanchet 2011). The efficiency of adding these types of particles to a coating varies depending on the particle size and a synergetic effect is possible when inorganic and organic particles are combined.

Drying oils are among the oldest binders used for paints. They are liquid vegetable or fish oils that react with oxygen in the air to form solid films. Such oils,  e.g . linseed oil, can also be used to impregnate wood in order to form a hydrophobic layer inside and outside the wood to retard the moisture uptake, and hence protect the wood substrate (Fredriksson  et al.  2010). Despite the advantage of vegetable and fish oils being a renewable resource, the use of drying oils as pure binders has decreased over the years. However, they still play a large role as raw materials for other binders such as alkyds and epoxy esters. Natural oils are triglycerides, consisting of glycerol and different fatty acids, such as stearic acid and oleic acid. The composition is different for each type of oil, and can also be affected by the growth location of the plant. The ability of oils to act as drying, semi-drying, or non-drying components depends on the composition of different fatty acids and their unsaturation level, or more specifically, the number of diallylic methylene groups in the chain. These natural oils can be modified by introducing new groups to the fatty acid chains in order to increase the compatibility with a potential top coating system or even to get the oils to react with the substrate. One example of this is to introduce epoxy groups to the fatty acid chains by, for example, lipase catalyzed  in situ  epoxidation (Vlcek and Petrovic 2006), in order to get other types of reactive groups on the fatty acids. These epoxy groups can then react covalently with the substrate, in order to create a better coating system due to less leaching (Kiguchi and Evans 1998; Olsson  et al.  2012).

Wood Coating with Epoxidized Vegetable Oils

Coatings present a physical barrier that protects the wood substrate from the adverse effect of environmental factors, such as solar irradiation, moisture, as well as staining and decay fungi. Clear coatings with high transparency, however, require additional UV absorbers and a radical scavenger, such as hindered amine light stabilizers to protect the coating itself and the wood surface underneath (Ahola 1991). Accordingly, it is assumed that certain pre-treatments of the wood substrate can improve the performance of exterior wood coatings providing dimensional stabilization, a reduction in capillarity, and a greater fungal resistance.

Epoxidized vegetable oils and their derivatives have been used for many comer-cial applications, e.g . , as plasticisers and stabilisers in chlorine-containing resins, as additives in lubricants, as components in thermosetting plastics, in cosmetics and pharmaceutical formulations, in urethane foams, and as wood impregnants (Wu  et al.  2000). Epoxy fatty acid compounds are obtained on an industrial scale mainly by the peracid process (Rangarajan  et al.  1995; Sinadinovic-Fiseret  et al.  2001; Hill 2000). Soybean oil is a triglyceride that typically contains 14% stearic, 23% oleic, 55% linoleic, and 8% linolenic acid. Chemical modification of commercially available soybean oil, such as epoxidation, can enhance its properties (reactivity) for certain industrial applica-tions. The epoxidized soybean oil (ESO), an epoxidized glycerol fatty ester with the structure given in Fig. 5, is extensively used in the plastic industry as a plasticizer to increase flexibility in poly (vinyl chloride) (PVC) products and as a stabilizer to minimize decomposition.

Fig. 5.  Chemical structure of epoxidized soybean oil (ESO)

Wood originating from a softwood species has been chemically modified by reaction with succinic anhydride in dimethylformamide (DMF) (Matsuda 1996). Modified wood samples were further coated with epoxidized soybean oil (ESO) in the presence of triethylamine (TEA). Synthesis of ESO was presented in a previous work (Mustaţă  et al.  2011). The photostability of modified wood and ESO-coated modified wood during artificial light irradiation was investigated (Teacă  et al.  2010; Teacă  et al.  2012). Analysis of the color changes in the wood surfaces was carried out by measuring CIELAB parameters. Structural changes were analyzed by Fourier transform infrared spectroscopy. The color difference, yellowing index, and weight loss were also evaluated. The FTIR spectrum of the non-modified wood sample (A) presented characteristic bands of wood (Fig. 6). In the spectrum of the modified wood sample with succinic anhydride (spectrum B) the absorbance intensity at 3330 cm -1  decreased and enlarged to the lower wavelength and the band at 1736 cm -1  significantly increased in intensity. The spectral changes confirmed the wood esterification with succinic anhydride. In the FTIR spectrum C, the signals at 2918, 2890, and 1736 cm -1  increased due to the reaction between the succinic monoester with ESO.

Fig. 6.  FTIR spectra recorded for the non-modified wood sample (A), wood modified with succinic anhydride (B), and wood sample modified with succinic anhydride and coated with ESO (C)

The FTIR spectra recorded for wood modified with SA before and after irradiation (exposure times of 200h and 400h, respectively) are shown in Fig. 7.

Wood samples modified with SA and coated with ESO were also investigated by FTIR spectroscopy for evidence of chemical changes after UV light exposure for different time values (Fig. 8).

As can be observed in Fig. 8, absorption bands at 2925 cm -1  and 2855 cm -1  increase as intensity with irradiation time exposure, while the band at 1509 cm -1  specific to aromatic skeletal vibration decreases during irradiation time, which can be attributed to the lignin degradation process (Hon 2001). Nevertheless, this evolution can be also due to the increasing roughness of wood surface. The absorption band at 1509 cm -1  is also noticed for wood samples modified with SA and coated with ESO in comparison with non-irradiated sample.

Absorption bands at 3419 cm -1  and 1732 cm -1 ,   as well   as those at 2925 cm -1  and 2855 cm -1 , significantly increase in intensity as irradiation time exposure increases. Analysis of color changes at coated wood surfaces shows that lightness (Δ L *) decreases whereas  a *,  b *, and Δ E * increases with increasing irradiation time. The rate of color change is very high during initial period of exposure. Total color change (Δ E *) is related with rate of formation of carbonyl groups and degradation of lignin (Hon 2001). Changes in wood color reflect chemical changes in its structure during irradiation. Yellowing of wood surfaces indicate modification of lignin and hemicelluloses with occurrence of unstable products. These chromophores are different, as color depends on the irradiation time and coating modified wood with ESO.

Fig. 7.  FTIR spectra recorded for wood treated with SA 80%: a- non-irradiated; b- irradiation 200 h; c- irradiation 400 h

Fig. 8.  FTIR spectra recorded for wood treated with SA 80% and further coated with ESO: a- non-irradiated; b- irradiation 200 h; c- irradiation 400 h

From Fig. 9, it is apparent that the increase of  E ab  values with irradiation time. There is a high rate of increase of  E ab  that is characteristic of the non-modified wood sample compared with the samples representing modified wood coated with ESO. The Δ L * ab  index of the samples significantly decreases as a result of photochemical aging (Pandey 2005b).

Fig. 9.  Total color difference of modified wood coated with ESO as a function of irradiation time (Teacă  et al.  2012)

Figure 10 presents the variation of  L * ab  with irradiation time. From Fig. 10, one can observe the blackening of the wood samples after irradiation. The  L ab *  values decrease during irradiation. The wood samples, modified with SA and coated with ESO, exhibited greater stability to blackening.

Fig. 10.  Lightness factor of modified wood coated with ESO as a function of irradiation time (Teacă  et al.  2012)

The positive values of  a *  (Fig. 11) show a progressive accumulation of red chromophores on the surface of the samples during irradiation. At irradiation times longer than 100 hours, the accumulation of red chromophores is less evidenced on the surface of coated softwood samples compared with the surface of reference sample. Less colored is the modified wood sample after coating with ESO. The increase in the chromaticity coordinates,  a * and  b * can be attributed to the formation of quinones and quinoide-like structures due to depolymerisation and oxidation of lignin involving free radicals (presumably phenoxyl radicals) in this process (Hon 2001).

Fig. 11.  Redness factor ( a * ) and yellowness factor ( b *) variation of modified wood coated with ESO with the irradiation time (Teacă  et al.  2012)

In the first 50 hours of irradiation, some blue chromophores were accumulated in all studied samples. By extension of irradiation time to 200 hours the blue chromophores are destroyed and yellowness shows systematic trends with increasing of the irradiation time. There is a slow increase of  b *  values for modified wood samples coated with ESO.

Wood Chemical Modification

In order to obtain a wood-based material with a long service life it is necessary to interfere with the natural degradation process of wood as much as possible. One way of achieving this is to modify the wood surface chemically by different treatments. Thus, the wood polymers (cellulose, hemicelluloses, and lignin) are modified, which leads to a change in the wood properties. To increase water repellency, for example, one way would be to reduce the hydrophilic character of the wood cell wall by introducing hydrophobic groups. Other types of chemical modifications are acetylation, furfurylation,  etc . (Rowell 2012). The basis of these modifications is OH-substitution of the wood components, in which the aliphatic and aromatic hydroxyl groups of the lignin are substituted by new groups. Apart from acetylation and furfurylation, another method is to graft UV protect-tive additives onto the wood surface by covalently bonding the epoxy- or isocyanate functionalized additives to the hydroxyl groups of the wood. In these types of grafting reactions it is of course desirable to get the grafting on the OH groups of the lignin, since these are the most susceptible to UV degradation. All of these modification methods can prevent the formation of phenoxy radicals, and thus the susceptibility to photo-degrada-tion is decreased (George  et al.  2005).

Greater wettability of weathered wood was suggested as a contributing factor to the deterioration of wood structures (Kalnins and Feist 1993). Previous researchers have demonstrated that lignocellulosic material properties (such as dimensional instability due to moisture and low durability due to biodegradation) can be improved by chemical modification using esterification (Efanov 2001; Hon and Xing 1992; Sereshiti and Rovshandeh 2003), etherification (Hon and Ou 1989; Norimoto  et al.  1983), and cyanoethylation (Hon and San Luis 1989; Liga  et al.  1995) reactions. Lignocellulosic materials are favored as a new generation reinforcing materials in thermoplastics since they represent renewable natural resources. Considering the increasing environmental concerns and the urge to promote recyclable raw materials and products, an emphasizing demand for lignocellulosic-thermoplastic composite materials (Mahlberg  et al.  2001) is obvious. Chemical modification of wood is defined as chemical reactions involving functional groups of wood polymers and a simple single chemical reagent that form covalent bonds (Rowell 1991; Kumar 1994). Dicarboxilic acid anhydrides such as phthalic (PA), maleic (MA), and succinic (SA) anhydrides have been used to esterify the lignocellulosic materials to obtain thermoformable products (Hassan  et al.  2000).

Lignocellulosic materials may be used as filler for polymer materials, which are characterized through increased water and fire resistance in comparison with wood (Rowell 1991; Lee  et al.  2000). An attractive option for outdoor wood use is to prevent its photo-degradation through chemical modification by esterification or etherification (Matsuda 1996; Chang and Chang 2002; Chang and Chang 2006). Wood esterification has primarily focused on the use of anhydrides (Chang and Chang 2001), or fatty acids and acid chlorides (Prakash and Mahadevan 2008), to improve dimensional stability, preservation, wood thermoplastic-matrix compatibility, and photo-stability. Although this subject has been investigated extensively (Evans 2009), relevant studies on photo-discoloration of modified wood surfaces are of great importance to protect products against weathering.

The esterification reaction applied to lignocellulosic materials may use different linear and cyclic anhydrides. The reaction between wood and linear anhydrides is a single site reaction yielding the corresponding carboxylic acid as a by-product of its reaction with wood. Figure 12 presents the reaction between wood and organic anhydrides (maleic and phthalic).

Fig. 12.  Wood chemical modification by reaction with organic anhydrides

Roşu  et al.  (2010) demonstrated that succinic anhydride (SA)-modified softwood can be slightly more stable to the artificial light action than non-modified wood. The light with > 300 nm leads to significant changes in the structure of softwood samples.

Structural changes in softwood after chemical modification evidenced by FTIR spectroscopy are shown in Fig. 13.

Fig. 13.  FT-IR spectra: (A) etalon sample   ( Abies alba  wood); (B)  Abies alba  wood sample esterified with succinic anhydride – adapted (Roşu  et al.  2010)

The FTIR spectrum of non-modified wood samples (A) presents characteristic bands of wood. In the spectrum of the modified wood sample with succinic anhydride (spectrum B) the absorbance intensity at 1726 cm -1  significantly increases, evidencing wood reaction with succinic anhydride.

Figure 14 shows the difference between the absorbances of the non-irradiated softwood sample and the absorbances recorded after 200 h of irradiation from the FTIR spectra. The difference spectrum shows a positive signal at 1509 cm -1 , which can be assigned to the partial decomposition of lignin. The decomposition of lignin is also indicated by the reduction of the signals in the range 2300 to 1900 cm -1 .

The intensity of bands characteristic to the carbonyl groups with aliphatic structures at 1726 cm -1  and with aromatic conjugated ketones such as quinones at 1630 cm -1  were enhanced significantly with increasing irradiation time. UV light results in the decrease of the lignin content. Some chromophores with carbonyl groups are formed through lignin degradation.

In Fig. 14 the negative absorbance values correspond to the new structures that were formed during irradiation. The peaks from 3300 cm -1  to 3000 cm -1 and also at 1729 cm -1 , 1627 cm -1 , 1595 cm -1 , 1431 cm -1 , 1313 cm -1 , 1238 cm -1 , 1107 cm -1 , and 1056 cm -1  are signals corresponding to the oxidized lignin.

Fig. 14.  FT-IR difference spectrum between the absorbance of non-irradiated, non-modified wood and the absorbance of wood recorded after 200 h of irradiation time – adapted (Roşu  et al.  2010)

Changes in the lignin content during irradiation were monitored from the FTIR signals area ratio [( A 1509 / A 897 ) t ]/[( A 1509 / A 897 ) 0 ] where ‘‘t” corresponds to the irradiated sample at different irradiation times and ‘‘0” corresponds to the non-irradiated sample. The photo-chemical decomposition of lignin in the non-modified and modified wood sample decreases as a function of irradiation time after an exponential law as shown in Fig. 15.

Fig. 15.  Variation of lignin content as a function of irradiation time measured by the ratio between the absorbance values at 1509 cm -1  ( A 1509 ) and 897 cm -1  ( A 897 ) for non-modified and modified wood samples – adapted (Roşu  et al.  2010)

Photo-chemical stability of the modified wood slightly increases with the increase in concentration of the succinic anhydride solution used in the esterification reaction. The decrease in photo-degradation in terms of reduced color changes on the surface of modified wood might be due to a slight increase in lignin stability to the polychromatic light action, induced by the esterification of wood with succinic anhydride (Roşu  et al.  2010). Color changes and the lightness factor variation during irradiation are presented in Fig. 16 and Fig. 17, respectively.

Fig. 16.  Color change as a function of irradiation time: (■) non-extracted wood; (o) extracted, non-modified wood; () wood modified with 10 g/L SA; () wood modified with 30 g/L SA; (□) wood modified with 60 g/L SA (Teacă  et al.  2010)

Fig. 17.  Dependence of lightness factor on the irradiation time: (■) non-extracted wood; (o) extracted, non-modified wood; () wood modified with 10 g/L SA; () wood modified with 30 g/L SA; (□) wood modified with 60 g/L SA (Teacă  et al.  2010)

The negative value of  L * for non-modified wood indicates that wood surface becomes darkened due to the UV light irradiation. The darkening of light irradiated surfaces of the non-modified wood might be due to degradation of lignin and other non-cellulosic polysaccharides (Hon and Chang 1984). The lignin decomposition during irradiation was delayed for esterified wood samples. The increase in esterification degree provided a better protection of wood exposed to artificial light.

CONCLUSIONS

  • Wood, a naturally occurring polymer composite, is the most versatile and widely used structural engineering material for indoor and outdoor applications. Due to its biological nature and aesthetic features, unprotected wood is susceptible to weathering and photo-oxidative degradation.
  • Deterioration of wood materials upon weathering involves a very complex process. Wood surface reactions initiated or accelerated by light can be manifested as discoloration, loss of brightness, and change in surface texture after artificial UV light irradiation or long-term solar irradiation.
  • The rate of discoloration depends upon light intensity and wavelength, and wood species. Changes in wood color reflect chemical changes during UV light irradiation. All wood polymers (cellulose, lignin, hemicelluloses) are capable of absorbing enough UV and VIS light to initiate wood photo-degradation, leading ultimately to changes in aesthetic, physical, chemical, and mechanical properties.
  • In order to prevent wood photo-induced discoloration, one or a combination of the following methods may be efficient as follows: cutting off UV light (by using UV absorbers), modification of wood light absorbing structures (by chemical treatments), destroying the structures involved in discoloration ( e.g . acetylation combined with oxidative bleaching and treatment with NaBH 4 ), cutting off oxygen and capturing the singlet oxygen ( e.g.  by using wood in wood-plastic composites, or using various quenchers), scavenging free radicals (by using compounds with active hydrogen such as phenolic derivatives and phenolic amines), and use of solvents to extract and remove precursors to discoloration.
  • Our previous work has provided evidence that succinic anhydride modified wood becomes more stable to the artificial light action than non-modified wood. Modified wood samples have been further investigated after coating with epoxy functionalized soybean oil (ESO). Yellowing of wood surfaces indicates that lignin and hemicelluloses are modified, with occurrence of unstable products. The resulting chromophores vary in color, depending on the irradiation time and with the modification of the wood by coating with ESO.

ACKNOWLEDGEMENTS

This work was supported by a grant from the Romanian National Authority for Scientific Research, CNCS-UEFISCDI – Project number PN-II-ID-PCE-2011-3-0187.

REFERENCES CITED

Ahola, P. (1991). “Moisture transport in wood coated with joinery paints,”  Holz Roh- Werkst.  49, 428-432.

Barata, M. A. B., Neves, M. C., Pascoal Neto, C., and Trindade, T. (2005). “Growth of BiVO4 particles in cellulosic fibres by in situ reaction,”  Dyes Pigm.  65, 125-127.

Barta, E., Tolvaj, L., Papp, G., Nagy, T., Szatmari, S., and Berkesi, O. (1998). “Wood degradation caused by UV-laser of 248 nm wavelength,”  Holz Roh Werkst.  56, 318.

Barta, E., Papp, G., Preklet, E., Tolvaj, L., Berkesi, O., Nagy, T., and Szatmari, S. (2005). “Changes of absorption in infrared spectra of softwood materials irradiated by UV-laser as a function of energy,”  Acta Silv. Lign. Hungarica  1(1), 83-91. (http://www.aslh.nyme.hu/)

Blanchard, V., and Blanchet, P. (2011). Color stability for wood products during use: Effects of inorganic nanoparticles,”  BioResources  6(2), 1219-1229.

Chang, S. T., and Chou, P. L. (1999). “Photo-discoloration of UV-curable acrylic coatings and the underlying wood,”  Polym. Degrad. Stab.  63, 435-439.

Chang, S. T., and Chang, H. T. (2001). “Comparisons of the photostability of esterified wood,”  Polym. Degrad. Stab.  71, 261-266.

Chang, H. T., and Chang, S. T. (2002). “Moisture excluding efficiency and dimensional stability of wood improved by acylation,”  Biores. Technol.  85, 201-204.

Chang, H. T., and Chang, S. T. (2006). “Modification of wood with isopropyl glycidyl ether and its effects on decay resistance and light stability,”  Biores. Technol.  97, 1265-1271.

Chaochanchaikul, K., Jayaraman, K., Rosarpitak, V., and Sombatsompop, N. (2012). “Influence of lignin content on photodegradation in wood/HDPE composites under UV weathering,”  BioResources  7(1), 38-55.

Chen, F. N., Yang, X. D., and Wu, Q. (2009). ”Antifungal capability of TiO 2  coated film on moist wood,”  Build. Environ.  44, 1088-1093.

Cui, W., Kamdem, D., and Rypstra, T. (2004). “Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) and color changes of artificial weathered wood,”  Wood Fiber Sci.  36, 291-301.

Derbyshire, H., and Miller, E.R. (1981). “The photodegradation of wood during solar irradiation. Part 1: Effects on the structural integrity of thin wood strips,”  Holz Roh-Werkst.  39(8), 341-350.

Efanov, M. V. (2001). “Cellulose esters prepared by wood esterification,”  Chem. Nat.  Comp.  37(1), 76-79.

Evans, P. D., Michell, A. J., and Schmalzl, K. J. (1992). “Studies of the degradation and protection of wood surfaces,”  Wood Sci. Technol.  26, 151-163.

Evans, P. D., Schmalzl, K. J., and Michell, A. J. (1993). “Rapid loss of lignin at wood

surfaces during natural weathering,” In: J. F. Kennedy, G. O. Phillips, P. A. Williams (eds.),  Cellulosics: Pulp, Fibre and Environmental Aspects , Ellis Horwood, Chichester, Chap. 51, pp. 335-340.

Evans, P. D. (2009). “Review of the weathering and photostability of modified wood,”  Wood Mat. Sci. Eng.  4(1), 2-13.

Evans, P. D., and Chowdhury, M. J. A. (2010). “Photoprotection of wood using polyester-type UV-absorbers derived from the reaction of 2-hydroxy-4(2, 3-epoxypropoxy)-benzophenone with dicarboxylic acid anhydrides,”  J. Wood Chem.Technol.  30(2), 186-204.

Evans, P. D. (2012). “Weathering of wood and wood composites,” In: R. M. Rowell (ed.),  Handbook of Wood Chemistry and Wood Composites , 2 nd Edition, CRC Press (Taylor & Francis Group), New York, Chap. 7, pp. 151-216.

Evans, P. D., Kraushaar Gibson, S., Cullis, I., Liu, C., and Sèbe, G. (2012). “Photostabilization of wood using low molecular weight phenol formaldehyde resin and hindered amine light stabilizer,”  Polym. Degr. Stab. , in press, doi: 10.1016/j.polymdegradstab.2012.10.015.

Fabiyi, J. S., McDonald, A. G., Wolcott, M. P., and Griffiths, P. R. (2008). “Wood plastic composites weathering: Visual appearance and chemical changes,”  Polym. Degrad. Stab.  93(8), 1405-1414.

Feist, W. C., and Hon, D. N.-S. (1984). “Chemistry of weathering and protection,” In: R. Rowell (ed.),  The Chemistry of Solid Wood , ACS Advances in Chemistry Series No. 207, p. 401.

Forsthuber, B., and Grüll, G. (2010). “The effects of HALS in the prevention of photo-degradation of acrylic clear topcoats and wooden surfaces,”  Polym. Degrad. Stab.  95(5), 746-755.

Fredriksson, M., Wadsö, L., and Ulvcrona, T. (2010). “Moisture sorption and swelling of Norway spruce [ Picea abies  (L.) Karst.] impregnated with linseed oil,”  Wood Mat. Sci. Eng.  5(3), 135-142.

Furuno, T., Imamura, Y., and Kajita, H. (2004). “The modification of wood by treatment with low molecular weight phenol-formaldehyde resin: a properties enhancement with neutralized phenolic-resin and resin penetration into wood cell walls,”  Wood Sci. Tech.  37(5), 349-361.

George, B., Suttie, E., Merlin, A., and Deglise, X. (2005). “Photo degradation and photo stability of wood—the state of art,”  Polym Degrad. Stab.  88(2), 268-274.

Hassan, L. M., Rowell, R. M., Fadl, N. A., Yacoub, S. F., and Christiansen, A. W. (2000). “Thermoplasticization of bagasse. I. Preparation and characterization of esterified bagasse fibers,”  J. Appl. Polym. Sci.  76, 561-574.

Hayoz, P., Peter, W., and Rogez, D. (2003). “A new innovative stabilization method for the protection of natural wood,”  Prog. Org. Coat.  48(2-4), 297-309.

Hill, K. (2000). “Fats and oils as oleochemical raw materials,”  Pure Appl. Chem.72,  1255-1264.

Hon, D. N. S., and Ifju, G. (1978). “Measuring penetration of light into wood by detection of photo-induced free radicals,”  Wood Sci.   Technol.  11,118-127.

Hon, D. N. S. (1983). “Weathering reactions and protection of wood surfaces,”  J. Appl.  Polym. Sci.  37, 845-864.

Hon, D. N.-S., and Chang, S.-T. (1984). “Surface degradation of wood by ultraviolet light,”  J. Polym. Sci. Polym. Chem.  22, 2227-2241.

Hon, D. N. S., and Feist, W. C. (1986). “Weathering characteristics of hardwood surfaces,”  Wood Sci. Technol.  20, 169-183.

Hon, D. N. S., and Ou, N. H. (1989). “Thermoplasticization of wood I. Benzylation of wood,”  J. Polym. Sci. Part A Polym. Chem . 27, 2457-2482.

Hon, D. N. S., and San Luis, J. M. (1989). “Thermoplasticization of wood. II. Cyanoethylation,”  J. Polym. Sci. Part A Polym. Chem . 27, 4143-4160.

Hon, D. N.-S. (1991). “Photochemistry of wood,” In  Wood and Cellulosic Chemistry , D. N.-S. Hon, and N. Shiraishi (eds.), Marcel Dekker, New York, pp. 525-556.

Hon, D. N. S., and Xing, L. M. (1992). In  Thermoplasticization of Wood by Esterification ; ACS Symposium Series 489; American Chemical Society: Washington DC., p.118.

Hon, D. N. S. (1994). “Degradative effects of ultraviolet light and acid rain on wood surface quality,”  Wood Fiber Sci . 26(2), 185-191.

Hon, D. N. S. (2001). “Weathering and photochemistry of wood,” In:  Wood and Cellulosic Chemistry , D. N.-S. Hon, and N. Shiraishi (eds.), 2 nd  Edition, Marcel Dekker, New York, pp. 512-546.

Horn, B. A., Qiu, J., Owen, N. L., and Feist, W. C. (1994). “FT-IR studies of weathering effects in western redcedar and southern pine,”  Appl. Spectrosc. 48, 662-668.

Kalnins, M. A. (1966). “Surface characteristics of wood as they affect durability of finishes. Part II. Photochemical degradation of wood,” U.S. Forest Service Research Paper FPL 57: 23-61.

Kalnins,   M., and   Feist ,  W.C. (1993) .  “Increase in wettability of wood with weathering,”  For. Prod. J.  43, 55-57.

Kataoka, Y., and Kiguchi, M. (2001). “Depth profiling of photo-induced degradation in wood by FT-IR microspectroscopy,”  J. Wood Sci.  47, 325-327.

Kataoka, Y., Kiguchi, M., and Evans, P. D. (2004). “Photodegradation depth profile and penetration of light in Japanese cedar earlywood ( Cryptomeria japonica  D. Don) exposed to artificial solar radiation,”  Surf. Coatings Int. Part B-Coatings Trans.  87(3), 187-193.

Kataoka, Y., Kiguchi, M., Fujiwara, T., and Evans, P. D. (2005). “The effects of within- species and between-species variation in wood density on the photodegradation depth profiles of sugi ( Cryptomeria japonica ) and hinoki ( Chamaecyparis obtusa ),”  J. Wood Sci.  51(5), 531-536.

Kataoka, Y., Kiguchi, M., Williams, R. S., and Evans, P. D. (2007). “Violet light causes photodegradation of wood beyond the zone affected by ultraviolet radiation,”  Holzforschung  61(1), 23-27.

Kiguchi, M., and Evans, P.D. (1998). “Photostabilisation of wood surfaces using a grafted benzophenone UV absorber,”  Polym. Degrad. Stab.  61, 33-45.

Kim, G. G., Kang, J. A., Kim, J. H., Kim, S. J., Lee, N. H., and Kim, S. J. (2006). “Metallization of polymer through a novel surface modification applying a photocatalytic reaction,”  Surf. Coat. Technol.  201, 3761-3766.

Kishino, M., and Nakano, T. (2004). “Artificial weathering of tropical woods. Part 1: Changes in wettability,”  Holzforschung  58, 552-557.

Košiková, B., and Tolvaj, L. (1998). “Structural changes of lignin-polysaccharide complex during photodegradation of  Populus grandis ,”  Drev. Vysk.  43, 37-46.

Kubelka, P. J., and Munk, F. (1931). “Ein Beitrag zur Optik der Farbanstriche,”  Zeitschrift fur Technische Physik  11(a), 593-601.

Kubelka, P. J. (1948). “New contributions to the optics of intensely light-scattering materials, Part I, ”  J. Opt. Soc. Am.  38, 448-457.

Kumar, S. (1994). “Chemical modification of wood,”  Wood Fiber Sci.  26(2), 270-280.

Kuroda, A., Joly, P., Shibata, N., Takeshige, H., and Asakura, K. (2008). “An organic–inorganic hybrid composite as a coating agent,”  J. Am. Oil Chem. Soc.  85 (6), 549-553.

Leary, G. J. (1967). “The yellowing of wood by light,”  Tappi J . 50(1), 17-19.

Leary, G. J. (1968). “Photochemical production of quinoid structures in wood,”  Nature  217, 672-673.

Lee, H., Chen, G. C., and Rowell, R. M. (2000). “Chemical modification of wood to improve decay and thermal resistance,” In:  Proceedings of the 5th Pacific Rim Bio-based Composites Symposium ; 2000, December 10-13; Canberra, Australia: Department of Forestry, The Australian National University, 179-189.

Li, J., Yu, H., Sun, Q., Liu, Y., Cui, Y., and Lu, Y. (2010). “Growth of TiO 2  coating on wood surface using controlled hydrothermal method at low temperatures,”  Appl. Surf. Sci . 256, 5046-5050.

Liga, A., Toma, C., Caranfil, A., Nita, M., and Rusu, Gh. (1995). “Chemical modification of wood by cyanoethylation,”  Rev. Roum. Chim.  40(7-8), 743-749.

Mahlberg, R., Paajanen, L., Nurme, A., Kivistö, A., Koskela, K., and Rowell, R. M. (2001). “Effect of chemical modification of wood on the mechanical and adhesion properties of wood fiber/polypropylene fiber and polypropylene/veneer composites,”  Holz.als Roh-und Werkstoff  59, 319-326.

Mahltig, B., Swaboda, C., Roessler, A., and Böttcher, H. (2008). “Functionalising wood by nanosol application,”  J. Mater. Chem.  18, 3180-3192.

Matsuda, H. (1996). “Chemical modification of solid wood,” In:  Chemical Modification  of Lignocellulosic Materials ,   D. N. S. Hon (ed.), Marcel Dekker, Inc., New York, pp. 159-183.

Matuana, L. M., Kamdem, D. P., and Zhang, J. (2001). “Photoaging and stabilization of rigid PVC/wood-fiber composites,”  J. Appl. Polym. Sci.  80 ( 11), 1943-1950.

Matuana, L. M., and Kamdem, D. P. (2002). “Accelerated ultraviolet weathering of PVC/wood-flour composites,”  Polym. Eng. Sci.  42(8), 1657- 1666.

Mitsui, K., Takada, H., Sugiyama, M., and Hasegawa, R. (2001). “Changes in the properties of light-irradiated wood with heat treatment Part 1. Effect of treatment conditions on the change in color,”  Holzforschung  55, 601-605.

Mitsui, K., Murata, A., and Tolvaj, L. (2003). “Investigation of the change in the DRIFT spectra of light-irradiated wood with heat treatment,”  Holz Roh Werkst.  61, 82.

Moore, A. K., and Owen, N. L. (2001). “Infrared spectroscopic studies of solid wood,”  Appl. Spectrosc. Rev . 36, 65-86.

Muasher, M., and Sain, M. (2006). “The efficacy of photostabilizers on the color change of wood filled plastic composites,”  Polym. Degrad. Stab.  91(5), 1156-1165.

Müller, U., Rätzsch, M., Schwanninger, M., Steiner, M., and Zöbl, H. (2003). “Yellowing and IR-changes of spruce wood as result of UV-irradiation,”  J. Photochem. Photobiol. B: Biol.  69, 97-105.

Mustaţă, F., Tudorachi, N., and Roşu, D. (2011). “Curing and thermal behavior of resin matrix for composites based on epoxidized soybean oil/diglycidyl ether of bisphenol A,”  Composites: Part B  42, 1803-1812.

Ncube, E., and Meincken, M. (2010). “Surface characteristics of coated soft- and hardwoods due to UV-B ageing,”  Appl. Surf. Sci.  256, 7504-7509.

Ndiaye, D., Fanton, E., Morlat-Therias, S., Vidal, L., Tidjani, A., and Gardette, J.-L. (2008). “Durability of wood polymer composites: Part 1. Influence of wood on the photochemical properties,”  Compos. Sci. Technol.  68(13), 2779-2784.

Norimoto, M., Morooka, T., Aoki, T., Shiraishi, N., Yamada T., and Tanaka, F. (1983). “Chemical modification of wood by etherification,”  Wood Res. Tech. Notes  17, 181-190.

Norrstrom, H. (1969). “Color of unbleached sulfate pulp,”  Svensk Papperstidn . 72, 25-38.

Olsson, K.S., Johansson, M., Westin, M., and Östmark, E. (2012). “Grafting of 2-hydroxy-4(2, 3-epoxypropoxy)-benzophenone and epoxidized soybean oil to wood: reaction conditions and effects on the color stability of Scots pine,”  Polym. Degrad. Stab.  97, 1779-1786.

Owen, N. L., and Thomas, D. W. (1989). “Infrared studies of ‘‘Hard’’ and ‘‘Soft’’ woods,”  Appl. Spectrosc . 43, 451-455.

Pandey, K. K., and Theagarajan, K. S. (1997). “Analysis of wood surfaces and ground wood by diffuse reflectance (DRIFT) and photoacoustic (PAS) Fourier transform infrared spectroscopic techniques,”  Holz Roh Werkst.  55, 383-390.

Pandey, K. K. (1999). “A study of chemical structure of soft and hardwood and wood polymers by FTIR spectroscopy,”  J. Appl. Polym. Sci.  71, 1969-1975.

Pandey, K. K. (2005a). “A note on the influence of extractives on the photo-discolouration and photo-degradation of wood,”  Polym. Degrad. Stab.  87, 375-379.

Pandey, K. K. (2005b). “Study of the effect of photo-irradiation on the surface chemistry of wood,”  Polym. Degrad. Stab.  90, 9-20.

Papp, G., Preklet, E., Košiková, B., Barta, E., Tolvaj, L., Bohus, J., Szatmári, S., and

Berkesi, O. (2004). “Effect of UV laser radiation with different wavelengths on the spectrum of lignin extracted from hard wood materials,”  J. Photochem. Photobiol. A: Chem.  163, 187-192.

Papp, G., Barta, E., Tolvaj, L., Berkesi, O., Nagy, T., and Szatmári, S. (2005). “Changes in DRIFT spectra of wood irradiated by UV laser as a function of energy,”  J. Photochem. Photobiol. A: Chem.  173(2), 137-142.

Prakash, G. K., and Mahadevan, K. M. (2008). “Enhancing the properties of wood through chemical modification with palmitoyl chloride,”  Appl. Surf. Sci.  254, 1751-1756.

Rabek, J. F. (1995).  Polymer Photodegradation: Mechanisms and Experimental Methods,  Springer, Germany, 664 pp.

Rangarajan, B., Havey A., Grulke, E. A., and Culnan, P. D. (1995). “Kinetic parameters of a two – phase. Model for in situ epoxidation of soybean oil,”  J. Am. Oil Chem. Soc. 72, 1161-1169.

Roşu, D., Teacă, C. A., Bodîrlău, R., and Roşu, L. (2010). “FTIR and color change of the modified wood as a result of artificial light irradiation,”  J. Photochem. Photobiol. B: Biol.  99, 144-149.

Rowell, R. M. (1991). “Chemical modification of wood,” In: Hon, D. N.-S., and N. Shiraishi (eds.),  Wood and Cellulosic Chemistry , Marcel Dekker, Inc. New York, pp. 703-756.

Rowell, R. M. (2005). “Chemical modification of wood,” In: Rowell, R.M. (ed.),  Handbook of Wood Chemistry and Wood Composites , CRC Press, Boca Raton, Chap.14, pp. 381-420.

Rowell, R. M. (2012). “Chemical modification of wood,” In: Rowell, R.M. (ed.),  Handbook of Wood Chemistry and Wood Composites , 2 nd  Edition, CRC Press (Taylor & Francis Group), New York, Chap. 15, pp. 537-598.

Schaller, C., and Rogez, D. (2007). “New approaches in wood coating stabilization,”  J. Coat. Technol. Res.  4 (4), 401-409.

Schmalzl, K. J., Forsyth, C. M., and Evans, P. D. (2003). “Evidence for the formation of chromium (III) diphenoquinone complexes during oxidation of guaiacol and 2, 6-dimethoxyphenol with chromic acid,”  Polym. Degrad. Stab.  82(3), 399-407.

Schmalzl, K. J., and Evans, P. D. (2003). “Wood surface protection with some titanium, zirconium and manganese compounds,”  Polym. Degrad. Stab.  82, 409-419.

Sereshiti, H., and Rovshandeh, J. M. (2003). “Chemical modification of beech wood,”  Iran. Polym. J.  12(1), 15-20.

Sinadinovic-Fiser, S., Jankovic, M., and Petrovic, Z. S. (2001). “Kinetics of In situ epoxidation of soybean oil in bulk catalyzed by ion exchange resin,”  J. Am. Oil Chem. Soc.  78, 725-731.

Stark, N. M., Matuana,   L. M., and Clemons, C. M. (2004). “Effect of processing method on surface and weathering characteristics of wood-flour/HDPE composites,”  J. Appl. Polym. Sci.  93(3), 1021-1030.

Stark, N. M., and Matuana,   L. M. (2004a). “Surface chemistry and mechanical property changes of wood-flour/high-density-polyethylene composites after accelerated weathering,”  J. Appl. Polym. Sci.  94(6), 2263-2273.

Stark, N. M., and Matuana,   L. M. (2004b). “Surface chemistry changes of weathered HDPE/wood-flour composites studied by XPS and FTIR spectroscopy,”  Polym. Degrad. Stab.  86(1), 1-9.

Stark,   N. M. (2006). “Effect of weathering cycle and manufacturing method on performance of wood flour and high-density polyethylene composites,”  J. Appl. Polym. Sci.  100(4), 3131-3140.

Stark, N. M., and Matuana,   L. M. (2006). “Influence of photostabilizers on wood flour-HDPE composites exposed to xenon-arc radiation with and without water spray,”  Polym. Degrad. Stab.  91(5), 3048-3056.

Stark, N. M., and Matuana,   L. M. (2007). “Characterization of weathered wood-plastic composite surfaces using FTIR spectroscopy, contact angle, and XPS,”  Polym. Degrad. Stab.  92(10), 1883-1890.

Stark, N. M., and Mueller, S. A. (2008). “Improving the color stability of wood-plastic composites through fiber pre-treatment,”  Wood Fiber Sci. , 40(2), 271-278.

Sun, Q., Lu, Y., and Liu, Y. (2011). “Growth of hydrophobic TiO 2  on wood surface using a hydrothermal method, “ J. Mater. Sci.  46, 7706-7712.

Sun, Q., Lu, Y., Zhang, H., Yang, D., Wang, Y., Xu, J., Tu, J., Liu, Y., and Li, J. (2012). “Improved UV resistance in wood through the hydrothermal growth of highly ordered ZnO nanorod arrays,”  J. Mater. Sci.  47, 4457-4462.

Teacă, C. A., Roşu, D., Bodîrlău, R., and Roşu, L. (2010). “Photostability of wood coated with epoxidized soybean oil,” Proceedings of the 7 th  International Conference of the South Eastern Countries Chemical Societies – ICOSECS7, September 15–17, Bucharest – Romania, 330-337.

Teacă, C. A., Bodîrlău, R., and Roşu, D. (2012). “Characterization of irradiated wood after chemical modification with organic anhydride and coating with epoxidized vegetable oil,” Proceedings of the 7 th  MoDeSt Conference, September 2 – 6, Prague, Czech Republic, Abstract book, 374-375.

Tolvaj, L., and Faix, O. (1995). “Artificial ageing of wood monitored by DRIFT spectroscopy and CIE  L * a * b * color measurements. I. Effect of UV light,”  Holzforschung  49, 397-404.

Tolvaj, L., and Mitsui, K. (2004). “Surface preparation and direction dependence of DRIFT spectra of wood,”  Appl .Spectrosc.  58, 1137-1140.

Tolvaj, L., and Mitsui, K. (2005). “Light source dependence of the photodegradation of wood,”  J. Wood Sci.  51, 468-473.

Tolvaj, L. Mitsui, K., and Varga, D. (2011). “Validity limits of Kubelka–Munk theory for DRIFT spectra of photodegraded solid wood,”  Wood Sci. Technol.  45, 135-146.

Tshabalala, M. A., and Gangstad, J. (2003). “Accelerated weathering of wood surfaces coated with multifunctional alkoxysilanes by sol-gel deposition,”  J. Coat. Technol.  75, 37-43.

Tshabalala, M. A., and Sung, L. P. (2007). “Wood surface modification by in-situ sol-gel deposition of hybrid inorganic-organic thin films,”  J. Coat. Technol. Res.  4, 483-490.

Umemura, K., Yamauchi, H., Ito, T., Shibata, M., and Kawai, S. (2008). “Durability of isocyanate resin adhesives for wood. V. Changes of color and chemical structure in photodegradation,”  J. Wood Sci.  54, 289-293.

Vlcek, T., and Petrovic, Z. S. (2006). “Optimization of the chemoenzymatic epoxidation of soybean oil,”  J. Am. Oil Chem. Soc.  83(3), 247-252.

Wang, S. Y., and Lin, S. J. (1991). “The effect of outdoor environmental exposure on the main component of woods,”  Mokuzai Gakkaishi  37(10), 954-963.

Weiland, J. J., and Guyonnet, R. (2003). “Study of chemical modifications and fungi degradation of thermally modified wood using DRIFT spectroscopy,”  Holz Roh Werkst . 61, 216-220.

Williams, R. S. (2005). “Weathering of wood,” In:  Handbook of Wood Chemistry and Wood Composites , R. M. Rowell (ed.), CRC Press, Boca Raton, USA, pp. 142-185.

Williams, R. S. (2010). “Finishing of wood,” In:  Wood Handbook – Wood as an Engineering Material , General Technical Report FPL-GTR-190, Madison, WI: U.S. Department of Agriculture, Forest Service, Forest Products Laboratory, Chap.16, pp. 16-1 – 16-39.

Williams, R. S., Knaebe, M. T., Sotos, P. G., and Feist, W. C. (2001a). “Erosion rates of wood during natural weathering: Part I. Effect of grain angle and surface texture,”  Wood Fiber Sci . 33(1), 31-42.

Williams, R. S., Knaebe, M. T., and Feist, W. C. (2001b). “Erosion rates of wood during natural weathering: Part II. Earlywood and latewood erosion rates,”  Wood Fiber Sci . 33(1), 43-49.

Williams, R. S., Knaebe, M. T., Evans, J. W., and Feist, W. C. (2001c). “Erosion rates of wood during natural weathering: Part III. Effect of exposure angle on erosion rate,”  Wood Fiber Sci . 33(1), 50-57.

Wu, X., Zhang, X., Yang, S., Chen, H., and Wang, D. (2000). “The study of epoxidized rapeseed oil used as a potential biodegradable lubricant,”  J. Am. Oil. Chem. Soc.  77(5), 561-563.

Zanuttini, M., Citroni, M., and Martinez, M. J. (1998). “Application of diffuse reflectance infrared Fourier transform spectroscopy to the quantitative determination of acetyl groups in wood,”  Holzforschung  52, 263-267.

Article submitted: May 18, 2012; Peer review completed: August 23, 2012; Revised version received and accepted: January 24, 2013; Published: January 31, 2013.

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35 Incredible Lighthouses You Can Visit In The US

Posted: December 2, 2023 | Last updated: December 2, 2023

Originally built to guide the way for ships and mark the entrances of harbors, today lighthouses have become beloved landmarks in their own right. Found on a variety of shorelines, from the rugged rocks of the Pacific coast to remote Alaskan islands, these light stations are just as pretty as they are practical. Here, we round up the loveliest of the lot.

Shoreline landmarks

<p>Opened in 1872, this black-and-white striped wonder on the Cape Hatteras National Seashore stands at 156 feet (47.5m) tall and is one of the few remaining brick-built lighthouse towers in the US in operation today. A restoration project between 2009 and 2013 ensured that the spiraling 214-step stairway to the top can be climbed by visitors and you can find out about <a href="https://www.nps.gov/caha/planyourvisit/bils.htm">opening times and tours here</a>. </p>

35. Bodie Island Light Station, North Carolina

Opened in 1872, this black-and-white striped wonder on the Cape Hatteras National Seashore stands at 156 feet (47.5m) tall and is one of the few remaining brick-built lighthouse towers in the US in operation today. A restoration project between 2009 and 2013 ensured that the spiraling 214-step stairway to the top can be climbed by visitors and you can find out about opening times and tours here . 

<p>West Quoddy Head Light sits on the most eastern point of the US mainland, meaning visitors to this spot can be the first to see the sunrise. The original lighthouse was built in 1808 but the current 49-foot (15m) tall tower dates back to 1858. It’s the only lighthouse in the country to have this particular red-and-white, candy cane-stripe design.</p>

34. West Quoddy Head Light, Maine

West Quoddy Head Light sits on the most eastern point of the US mainland, meaning visitors to this spot can be the first to see the sunrise. The original lighthouse was built in 1808 but the current 49-foot (15m) tall tower dates back to 1858. It’s the only lighthouse in the country to have this particular red-and-white, candy cane-stripe design.

<p>With its precarious position on the rugged rocks of San Francisco Bay’s north side, Point Bonita Lighthouse is certainly exposed to the elements. Built in 1855, it was the fourth lighthouse on the West Coast at the time. Yet it hasn’t stayed in the same spot. Initially constructed on a cliff 300 feet (91m) above the ocean, people quickly pointed out that San Fran's notoriously high fog obscured the light. So in 1877 the lighthouse was moved to its current, lower and fog-free location on the southeastern tip of the headland, where it remains active, guiding mariners into the Golden Gate straits.</p>

33. Point Bonita Lighthouse, California

With its precarious position on the rugged rocks of San Francisco Bay’s north side, Point Bonita Lighthouse is certainly exposed to the elements. Built in 1855, it was the fourth lighthouse on the West Coast at the time. Yet it hasn’t stayed in the same spot. Initially constructed on a cliff 300 feet (91m) above the ocean, people quickly pointed out that San Fran's notoriously high fog obscured the light. So in 1877 the lighthouse was moved to its current, lower and fog-free location on the southeastern tip of the headland, where it remains active, guiding mariners into the Golden Gate straits.

<p>While it might look more like a charming house on the cliff, this is in fact a fully working lighthouse. Built in 1871, soon after the founding of the city of Newport, the quaint Yaquina Bay Lighthouse sits atop a steep bluff at the mouth of the Yaquina River and is alleged to be the oldest structure in the city. Decommissioned in 1874, it was restored as a privately maintained beacon in 1996. </p>  <p><a href="https://www.facebook.com/loveexploringUK?utm_source=msn&utm_medium=social&utm_campaign=front"><strong>Love this? Follow our Facebook page for more travel inspiration</strong></a></p>

32. Yaquina Bay Lighthouse, Oregon

While it might look more like a charming house on the cliff, this is in fact a fully working lighthouse. Built in 1871, soon after the founding of the city of Newport, the quaint Yaquina Bay Lighthouse sits atop a steep bluff at the mouth of the Yaquina River and is alleged to be the oldest structure in the city. Decommissioned in 1874, it was restored as a privately maintained beacon in 1996. 

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<p>A lighthouse has graced the shores of Aquinnah, Massachusetts since 1796 – although neither the structure nor the location has remained quite the same. After the original wooden tower decayed, a new brick version was built in 1854, whose long history has been preserved and celebrated by the Martha's Vineyard community ever since. In 2015, Gay Head Lighthouse had to be moved 135 feet (41m) southeast of its previous spot, to prevent it from toppling off the fast-eroding cliffs. </p>  <p><a href="https://www.loveexploring.com/galleries/103411/the-worlds-most-beautiful-coasts?page=1"><strong>Now check out the most beautiful coastlines on the planet</strong></a></p>

31. Gay Head Lighthouse, Massachusetts

A lighthouse has graced the shores of Aquinnah, Massachusetts since 1796 – although neither the structure nor the location has remained quite the same. After the original wooden tower decayed, a new brick version was built in 1854, whose long history has been preserved and celebrated by the Martha's Vineyard community ever since. In 2015, Gay Head Lighthouse had to be moved 135 feet (41m) southeast of its previous spot, to prevent it from toppling off the fast-eroding cliffs. 

Check out the most beautiful coastlines on the planet

<p>You get two for one at Virginia Beach. The ‘Old’ Cape Henry Lighthouse, pictured right, opened in 1792, and was the first federally funded public construction project by the newly formed US Government. It's near the ‘First Landing’ site where English settlers arrived. The black-and-white ‘New’ Cape Henry Lighthouse was built in 1881 and stands just 350 feet (107m) away from the original.</p>

30. Cape Henry Lighthouse, Virginia

You get two for one at Virginia Beach. The ‘Old’ Cape Henry Lighthouse, pictured right, opened in 1792, and was the first federally funded public construction project by the newly formed US Government. It's near the ‘First Landing’ site where English settlers arrived. The black-and-white ‘New’ Cape Henry Lighthouse was built in 1881 and stands just 350 feet (107m) away from the original.

<p>Built in 1848, Biloxi was one of the first cast-iron lighthouses in the South. It’s also the only lighthouse in the country to be located in the middle of a major highway. Standing at 64 feet (19.5m) tall, the lighthouse was in service until 1939. The tower has been battered by, and survived, many storms including Hurricane Katrina in 2005 and has become a symbol of resilience for the residents of Biloxi. </p>

29. Biloxi Lighthouse, Mississippi

Built in 1848, Biloxi was one of the first cast-iron lighthouses in the South. It’s also the only lighthouse in the country to be located in the middle of a major highway. Standing at 64 feet (19.5m) tall, the lighthouse was in service until 1939. The tower has been battered by, and survived, many storms including Hurricane Katrina in 2005 and has become a symbol of resilience for the residents of Biloxi. 

<p>First lit in 1874, the Block Island Southeast Light replaced a previous building that fell victim to Mother Nature and it’s one of the most sophisticated lighthouses built during the 19th century. Located at the southeastern corner of the six-mile (10km) long Block Island, the red-brick building has a 52-foot (15.8m) tall and 25-feet (7.6m) wide tower, topped by a 16-sided lantern room.</p>

28. Block Island Southeast Light, Rhode Island

First lit in 1874, the Block Island Southeast Light replaced a previous building that fell victim to Mother Nature and it’s one of the most sophisticated lighthouses built during the 19th century. Located at the southeastern corner of the six-mile (10km) long Block Island, the red-brick building has a 52-foot (15.8m) tall and 25-feet (7.6m) wide tower, topped by a 16-sided lantern room.

<p>This striking 175-foot (53m) tall red tower, located 10 miles (16km) south of Daytona Beach, is the tallest lighthouse in Florida. Built in the 1880s, it’s also one of the best-preserved light stations in the US and was designated a National Historic Landmark in 1998. Today, many visitors choose to climb the 203 steps to the top for incredible panoramic views across the Atlantic coast.</p>

27. Ponce de Leon Inlet Light Station, Florida

This striking 175-foot (53m) tall red tower, located 10 miles (16km) south of Daytona Beach, is the tallest lighthouse in Florida. Built in the 1880s, it’s also one of the best-preserved light stations in the US and was designated a National Historic Landmark in 1998. Today, many visitors choose to climb the 203 steps to the top for incredible panoramic views across the Atlantic coast.

<p>First illuminated in 1875, this Gothic-inspired brick tower looks out over the northern Outer Banks in Corolla village. Inside, there’s a 214-step spiral staircase to the top, which offers dazzling views of Currituck Sound, the Atlantic Ocean and the Currituck Outer Banks. </p>

26. Currituck Beach Lighthouse, North Carolina

First illuminated in 1875, this Gothic-inspired brick tower looks out over the northern Outer Banks in Corolla village. Inside, there’s a 214-step spiral staircase to the top, which offers dazzling views of Currituck Sound, the Atlantic Ocean and the Currituck Outer Banks. 

<p>Opened in 1879 on a tiny off-shore island, or 'nubble', in York, this Victorian charmer – lovingly known as Nubble Light and Cape Neck to locals – is still in use today. It includes an adjoining lighthouse keeper’s house, inhabited by a string of caretakers, several of whom were <a href="https://www.lighthousefriends.com/light.asp?ID=548">known to charge tourists</a> 10 cents to ferry them over to the island outside official visiting times. Although visitors can’t go onto the island today, it’s close to the shore and the nearby Sohier Park has become a prime spot for gazing out at the postcard-perfect scene. </p>

25. Cape Neddick Lighthouse, Maine

Opened in 1879 on a tiny off-shore island, or 'nubble', in York, this Victorian charmer – lovingly known as Nubble Light and Cape Neck to locals – is still in use today. It includes an adjoining lighthouse keeper’s house, inhabited by a string of caretakers, several of whom were known to charge tourists  10 cents to ferry them over to the island outside official visiting times. Although visitors can’t go onto the island today, it’s close to the shore and the nearby Sohier Park has become a prime spot for gazing out at the postcard-perfect scene. 

<p>Sitting atop a steep bluff along California's beautiful Highway 1, around 50 miles (80km) south of San Francisco, the 115-foot (35m) Pigeon Point Lighthouse is one of America’s tallest. It was first lit in 1872 and remains in operation today, although the original lens has been replaced by an automated LED and it’s only used for training purposes by the US Coast Guard. </p>  <p><strong><a href="https://www.loveexploring.com/guides/107799/ultimate-stops-on-the-pacific-coast-highway-california-road-trip-usa">Discover Highway 1's other incredible sights</a></strong></p>

24. Pigeon Point Lighthouse, California

Sitting atop a steep bluff along California's beautiful Highway 1, around 50 miles (80km) south of San Francisco, the 115-foot (35m) Pigeon Point Lighthouse is one of America’s tallest. It was first lit in 1872 and remains in operation today, although the original lens has been replaced by an automated LED and it’s only used for training purposes by the US Coast Guard. 

Discover Highway 1's other incredible sights

<p>This quirky three-story, 11-room brick light station sits in the Thames River at the mouth of New London's harbor on its own man-made island. It owes its unique appearance to the fact that two wealthy local homeowners created the structure in the image of their own grand houses. Completed in 1909, it was one of the last lighthouses to be built in New England and it's still in use today. </p>

23. New London Ledge Light, Connecticut

This quirky three-story, 11-room brick light station sits in the Thames River at the mouth of New London's harbor on its own man-made island. It owes its unique appearance to the fact that two wealthy local homeowners created the structure in the image of their own grand houses. Completed in 1909, it was one of the last lighthouses to be built in New England and it's still in use today. 

<p>The only lighthouse in the US with this distinctive barber-pole design, White Shoal Light was first lit in 1910 and stands at 121 feet (37m) tall. Located 20 miles (32km) west of the Mackinac Bridge in Lake Michigan, the beacon is currently poised for restoration. The plans include creating six private rooms to host up to 12 guests for overnight stays, as well as a museum, bar and a lounge in the former lantern. </p>  <p><a href="https://www.loveexploring.com/galleries/95160/americas-most-beautiful-lakes-in-pictures?page=1"><strong>Discover America's most beautiful lakes</strong></a></p>

22. White Shoal Lighthouse, Michigan

The only lighthouse in the US with this distinctive barber-pole design, White Shoal Light was first lit in 1910 and stands at 121 feet (37m) tall. Located 20 miles (32km) west of the Mackinac Bridge in Lake Michigan, the beacon is currently poised for restoration. The plans include creating six private rooms to host up to 12 guests for overnight stays, as well as a museum, bar and a lounge in the former lantern. 

Discover America's most beautiful lakes

<p>This cute little tower at Cape Meares, just south of Tillamook Bay, is inactive but still popular with visitors who can typically see inside on an organized tour during the summer months. Opened in 1890, it’s the shortest lighthouse on the Oregon coast at a dinky 38 feet (12m) tall. Poking out among the trees of a National Wildlife Refuge, the setting only adds to its quaint good looks. </p>

21. Cape Meares Lighthouse, Oregon

This cute little tower at Cape Meares, just south of Tillamook Bay, is inactive but still popular with visitors who can typically see inside on an organized tour during the summer months. Opened in 1890, it’s the shortest lighthouse on the Oregon coast at a dinky 38 feet (12m) tall. Poking out among the trees of a National Wildlife Refuge, the setting only adds to its quaint good looks. 

<p>Sitting on a small rocky outcrop just off the coast of Crescent City, Battery Point Lighthouse is only accessible at low tide. First lit in 1856, the light is still in operation today, while many of the original furnishings inside remain, giving a glimpse into the lives of former keepers and their families. </p>

20. Battery Point Lighthouse, California

Sitting on a small rocky outcrop just off the coast of Crescent City, Battery Point Lighthouse is only accessible at low tide. First lit in 1856, the light is still in operation today, while many of the original furnishings inside remain, giving a glimpse into the lives of former keepers and their families. 

<p>One of America’s prettiest lighthouses, the red-brick Split Rock is located southwest of Silver Bay on the North Shore of Lake Superior. The 54-foot (16.5m) tower was first lit in 1910 and was built after many maritime tragedies on the lake, culminating in the perilous gale of November 1905, when 18 ships were sunk or badly damaged in two days. Situated on towering cliffs and surrounded by its own state park with trails, waterways and campgrounds, the lighthouse is nothing short of majestic. </p>

19. Split Rock Lighthouse, Minnesota

One of America’s prettiest lighthouses, the red-brick Split Rock is located southwest of Silver Bay on the North Shore of Lake Superior. The 54-foot (16.5m) tower was first lit in 1910 and was built after many maritime tragedies on the lake, culminating in the perilous gale of November 1905, when 18 ships were sunk or badly damaged in two days. Situated on towering cliffs and surrounded by its own state park with trails, waterways and campgrounds, the lighthouse is nothing short of majestic. 

<p>Built in 1917 to guide shipping on Lake Erie, one of the US’ five Great Lakes, the picture-perfect Lorain lighthouse is no longer in operation – but it’s still much loved. The light's trustees and volunteers have ensured its legacy has not been forgotten by transforming it into a wonderful attraction, earning it the nickname “Jewel of the Port”. Sunset dinners and tours are still running during <a href="https://lorainlighthouse.com/upcomingevents/">the summer season</a> with COVID restrictions in place. </p>

18. Lorain Lighthouse, Ohio

Built in 1917 to guide shipping on Lake Erie, one of the US’ five Great Lakes, the picture-perfect Lorain lighthouse is no longer in operation – but it’s still much loved. The light's trustees and volunteers have ensured its legacy has not been forgotten by transforming it into a wonderful attraction, earning it the nickname 'Jewel of the Port'.

The super-cute Point Reyes Lighthouse feels very Californian. Located on the weather-worn headlands of Point Reyes in Marin County, the 16-sided lantern and 375-foot (114m) tower can only be accessed by climbing down the 313 stairs along the cliff. First lit in 1870, its parts were built in France and South America before being constructed on this spot, which had to be blown up with dynamite to level the surface.

17. Point Reyes Lighthouse, California

<p>It may be charming to look at but the 198-foot (60m) tall Cape Hatteras Lighthouse protects one of the most dangerous parts of the coastline known as “the Graveyard of the Atlantic”. The first tower that stood here was built in 1803, but it was too short and did not provide an effective signal, so it was replaced by the structure that stands today. First lit in 1870, the beacon gained its striking black-and-white striped pattern three years later.</p>

16. Cape Hatteras Lighthouse, North Carolina

It may be charming to look at but the 198-foot (60m) tall Cape Hatteras Lighthouse protects one of the most dangerous parts of the coastline known as “the Graveyard of the Atlantic”. The first tower that stood here was built in 1803, but it was too short and did not provide an effective signal, so it was replaced by the structure that stands today. First lit in 1870, the beacon gained its striking black-and-white striped pattern three years later.

<p>Boston Light on Little Brewster island has probably the most eventful history of any in the country. The first structure, made of rubblestone and wood, was built in 1716 and was lit by candles. It was burned down twice during the Revolutionary War, before being blown up by the retreating British in 1776. The Boston Light that stands today was rebuilt in 1783 and measures 75 feet (22.8m) tall. </p>

15. Boston Light, Massachusetts

Boston Light on Little Brewster island has probably the most eventful history of any in the country. The first structure, made of rubblestone and wood, was built in 1716 and was lit by candles. It was burned down twice during the Revolutionary War, before being blown up by the retreating British in 1776. The Boston Light that stands today was rebuilt in 1783 and measures 75 feet (22.8m) tall. 

<p>Sitting in the middle of the stormy waters of Lynn Canal, Eldred Rock Lighthouse was created after a series of shipwrecks in the late 1800s. The oldest Alaskan lighthouse that still stands, the octagonal-framed, pyramid-roofed building was designed to withstand stormy weather. Backdropped by snow-capped mountains, it’s a remarkable sight.</p>

14. Eldred Rock Lighthouse, Alaska

Sitting in the middle of the stormy waters of Lynn Canal, Eldred Rock Lighthouse was created after a series of shipwrecks in the late 1800s. The oldest Alaskan lighthouse that still stands, the octagonal-framed, pyramid-roofed building was designed to withstand stormy weather. Backdropped by snow-capped mountains, it’s a remarkable sight.

<p>Georgia’s historic Tybee Lighthouse, located at the entrance of the Savannah River, is both the oldest and tallest in the state. The first wooden lighthouse was built in 1736, but the current tower's lower portion dates from 1773, while an upper section was added in 1867. The charming site also encompasses other historic buildings including a museum and several cottages. </p>

13. Tybee Lighthouse, Georgia

Georgia’s historic Tybee Lighthouse, located at the entrance of the Savannah River, is both the oldest and tallest in the state. The first wooden lighthouse was built in 1736, but the current tower's lower portion dates from 1773, while an upper section was added in 1867. The charming site also encompasses other historic buildings including a museum and several cottages. 

<p>Constructed in 1825, Cape Florida Light is located at Key Biscayne near Miami. It was decommissioned in 1878 and put back into use in 1978 by the US Coast Guard, but damaged in 1992 by Hurricane Andrew. Now restored, today it forms part of the lovely Bill Baggs Cape Florida State Park.</p>  <p><strong><a href="https://www.loveexploring.com/galleries/86372/the-most-beautiful-state-park-in-every-us-state?page=1">Discover the most beautiful state park near you</a></strong></p>

12. Cape Florida Light, Florida

Constructed in 1825, Cape Florida Light is located at Key Biscayne near Miami. It was decommissioned in 1878 and put back into use in 1978 by the US Coast Guard, but damaged in 1992 by Hurricane Andrew. Now restored, today it forms part of the lovely Bill Baggs Cape Florida State Park.

Discover the most beautiful state park near you

<p>Finished in 1970, this 90-foot (27.4m) candy-striped, hexagonal lighthouse with a red observation deck is a mere baby compared to others on the list but no less charming for it. Privately built to watch over Hilton Head Island and Sea Pines Resort, it has become a much-loved landmark that features a museum, and gift shops at the bottom and top of the light. </p>

11. Harbour Town Lighthouse, Hilton Head, South Carolina

Finished in 1970, this 90-foot (27.4m) candy-striped, hexagonal lighthouse with a red observation deck is a mere baby compared to others on the list but no less charming for it. Privately built to watch over Hilton Head Island and Sea Pines Resort, it has become a much-loved landmark that features a museum, and gift shops at the bottom and top of the light. 

<p>Perched at the end of a rocky peninsula near the fishing village of Port Clyde, the 31-foot (9.4m) tall Marshall Point Lighthouse is connected to its keeper’s house by a long wooden walkway. Built in 1858 to replace a previous lighthouse, the white tower has been lovingly restored since. It’s also known for making a brief cameo in the movie <em>Forrest Gump</em> in 1994.</p>

10. Marshall Point Lighthouse, Maine

Perched at the end of a rocky peninsula near the fishing village of Port Clyde, the 31-foot (9.4m) tall Marshall Point Lighthouse is connected to its keeper’s house by a long wooden walkway. Built in 1858 to replace a previous lighthouse, the white tower has been lovingly restored since. It’s also known for making a brief cameo in the movie  Forrest Gump  in 1994.

<p>Looking ultra-atmospheric in this wintry shot, the 108-foot (32m) tall Wind Point Lighthouse on Lake Michigan is one of the tallest and oldest in the Great Lakes region. It was first lit in 1880 and originally contained huge fog horns, whose signals could be heard up to 10 miles (16km) away, as well as a huge Fresnel lens which is now kept in the Coast Guard Keepers Quarters. Listed on the National Register of Historic Places in 1980, the light is still in operation today.</p>

9. Wind Point Lighthouse, Wisconsin

Looking ultra-atmospheric in this wintry shot, the 108-foot (32m) tall Wind Point Lighthouse on Lake Michigan is one of the tallest and oldest in the Great Lakes region. It was first lit in 1880 and originally contained huge fog horns, whose signals could be heard up to 10 miles (16km) away, as well as a huge Fresnel lens which is now kept in the Coast Guard Keepers Quarters. Listed on the National Register of Historic Places in 1980, the light is still in operation today.

<p>The quaint Heceta Head Light is situated about halfway along Oregan's coastline and stands on an impressive 1,000-foot (305m) tall bluff above the crashing Pacific Ocean. The lighthouse was first lit in 1894 and it’s still guiding mariners to this day, using an automated light which is the most powerful rated on Oregon's coast. The keeper's cottage has been transformed into a B&B which offers unbeatable views of the cliffs and beach below too. </p>

8. Heceta Head Lighthouse, Oregon

The quaint Heceta Head Light is situated about halfway along Oregan's coastline and stands on an impressive 1,000-foot (305m) tall bluff above the crashing Pacific Ocean. The lighthouse was first lit in 1894 and it’s still guiding mariners to this day, using an automated light which is the most powerful rated on Oregon's coast. The keeper's cottage has been transformed into a B&B which offers unbeatable views of the cliffs and beach below too. 

<p>The oldest lighthouse in the state, Portland Head Light sits on the headland in Cape Elizabeth, keeping a watchful eye over the shipping lanes to Portland harbor. It was first illuminated in 1791, while the keeper’s house was constructed a century later and was home to the lighthouse's caretakers and their families until 1989. Today, the light station is managed by the town of Cape Elizabeth and there’s an award-winning museum in the keepers' quarters.</p>

7. Portland Head Light, Maine

The oldest lighthouse in the state, Portland Head Light sits on the headland in Cape Elizabeth, keeping a watchful eye over the shipping lanes to Portland harbor. It was first illuminated in 1791, while the keeper’s house was constructed a century later and was home to the lighthouse's caretakers and their families until 1989. Today, the light station is managed by the town of Cape Elizabeth and there’s an award-winning museum in the keepers' quarters.

<p>At more than 250 years old, Sandy Hook Lighthouse is the oldest operating lighthouse in the country. Located on a low-lying spit at the entrance of New York Harbor, it was completed in 1764. The light is part of the National Recreation Area and while tours are currently suspended it's still possible to enjoy the six miles (9.6km) of golden sands as well as bird-watching and fishing.</p>  <p><strong><a href="https://www.loveexploring.com/galleries/108596/americas-oldest-and-most-historic-attractions?page=1">These are America's oldest attractions</a></strong></p>

6. Sandy Hook Lighthouse, New Jersey

At more than 250 years old, Sandy Hook Lighthouse is the oldest operating lighthouse in the country. Located on a low-lying spit at the entrance of New York Harbor, it was completed in 1764. The light is part of the National Recreation Area and it's possible to enjoy the six miles (9.6km) of golden sands as well as bird-watching and fishing.

These are America's oldest attractions

<p>Recognize this quaint little lighthouse? Diamond Head Lighthouse in Oahu, Hawaii was memorialized on a US postage stamp in 2007. Sitting on the southwestern edge of Diamond Head, a 3,520-foot (1,073m) wide crater formed by a volcanic eruption 300,000 years ago, the lighthouse has stood guard over the coast since 1899, when it was built following two shipwrecks.</p>  <p><a href="https://www.loveexploring.com/gallerylist/69708/american-islands-that-arent-in-north-america"><strong>Discover these American islands that aren't in North America</strong></a></p>

5. Diamond Head Lighthouse, Hawaii

Recognize this quaint little lighthouse? Diamond Head Lighthouse in Oahu, Hawaii was memorialized on a US postage stamp in 2007. Sitting on the southwestern edge of Diamond Head, a 3,520-foot (1,073m) wide crater formed by a volcanic eruption 300,000 years ago, the lighthouse has stood guard over the coast since 1899, when it was built following two shipwrecks.

<p>Loggerhead Lighthouse is about as remote as they come. Which makes it all the more surprising that the 150-foot (46m) tall tower, located on Loggerhead Key in the Dry Tortugas islands – even further west than Key West – was the second lighthouse to be built in this archipelago. The first, Tortugas Harbor Light was erected in 1826 but by the mid-19th century, an increase in shipwrecks in the area led to the construction of Loggerhead Lighthouse in 1858. </p>  <p><a href="https://www.loveexploring.com/gallerylist/69708/american-islands-that-arent-in-north-america"><strong>Discover these American islands that aren't in North America</strong></a></p>

4. Loggerhead Lighthouse, Florida

Loggerhead Lighthouse is about as remote as they come. Which makes it all the more surprising that the 150-foot (46m) tall tower, located on Loggerhead Key in the Dry Tortugas islands – even further west than Key West – was the second lighthouse to be built in this archipelago. The first, Tortugas Harbor Light was erected in 1826 but by the mid-19th century, an increase in shipwrecks in the area led to the construction of Loggerhead Lighthouse in 1858. 

Discover these American islands that aren't in North America

Standing out against a vivid pink sunset, it’s easy to see why photographers love Round Island Light. Built in 1892 to shine a light across the channels between Round Island and Mackinac Island, it features a 57-foot (17m) brick tower topped with a hexagonal lantern room, plus adjoining keepers’ quarters. The light station was badly damaged by a storm in 1972, which chipped a section off the bottom, but thanks to extensive restoration its former charm has been revived.

3. Round Island Light, Michigan

The dinky, 46-foot (14m) tall Cockspur Island Lighthouse stands on an islet just southeast of Cockspur Island. The original brick tower was erected in the late 1830s but was obliterated by a hurricane, so in 1855 the present light station was built. It’s certainly survived harsh conditions, including multiple hurricanes and a siege during the Civil War. Restored between 1995 and 2000, today its light beam is solar-powered.

2. Cockspur Island Lighthouse, Georgia

<p>This 112-foot (34m) tower was built in 1867 on the eastern shore of Lake Michigan. It was the first lighthouse in the area but the site was largely abandoned after the last full-time keeper left in the late 1940s. The light features a distinctive black-and-white exterior, with 130 steps inside to reach the top, where visitors can gaze at the impressive views across both Ludington State Park and the lake. </p>  <p><strong><a href="https://www.loveexploring.com/galleries/107788/the-worlds-most-beautiful-lighthouses?page=1">Now discover the world's most beautiful lighthouses too</a></strong></p>

1. Big Sable Point Lighthouse, Michigan

This 112-foot (34m) tower was built in 1867 on the eastern shore of Lake Michigan. It was the first lighthouse in the area but the site was largely abandoned after the last full-time keeper left in the late 1940s. The light features a distinctive black-and-white exterior, with 130 steps inside to reach the top, where visitors can gaze at the impressive views across both Ludington State Park and the lake. 

Now read on to discover more facts and secrets about North America's Great Lakes

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  1. [Solved] Can light travel through solid substances such

    can light travel in wood

  2. Can Light Travel Through Wood

    can light travel in wood

  3. Light into wood

    can light travel in wood

  4. Making a wooden light

    can light travel in wood

  5. Light into wood

    can light travel in wood

  6. Light Travels in a Straight Line

    can light travel in wood

COMMENTS

  1. Can light travel through solid substances such as wood?

    Since sound can travel through wood (oak) at 3848 m/sec, does that mean that light can also travel through wood, but at a faster speed? I already knew that light was the ultimate speed limit, but I just came out of a study of sound, so this was a question that naturally popped up. I am not entirely certain of the nature of light, so I don't ...

  2. Does Light Travel Forever?

    Sound cannot travel through empty space; it is carried by vibrations in a material, or medium (like air, steel, water, wood, etc). As the particles in the medium vibrate, energy is lost to heat, viscous processes, and molecular motion. ... In contrast, light waves can travel through a vacuum, and do not require a medium. In empty space, the ...

  3. How does light travel?

    Basically, traveling at incredible speeds (299 792 458 m/s) and at different wavelengths, depending on its energy. It also behaves as both a wave and a particle, able to propagate through mediums ...

  4. How Does Light Travel?

    A Ray of Light. When an electromagnetic source generates light, the light travels outward as a series of concentric spheres spaced in accordance with the vibration of the source. Light always takes the shortest path between a source and destination. A line drawn from the source to the destination, perpendicular to the wave-fronts, is called a ray.

  5. Why Light Can't Travel Through Walls

    In this video, we explore the fascinating science behind why light can't travel through walls. We start by examining the structure of solid materials like co...

  6. Scientists develop transparent wood that is stronger and lighter than

    The epoxy in the wood does an even better job, allowing 90 per cent of visible light to pass through. The result is a long piece of what looks like glass, with the strength and flexibility of wood.

  7. Light: Electromagnetic waves, the electromagnetic spectrum and photons

    We can start with our equation that relates frequency, wavelength, and the speed of light. c = λ ν. Next, we rearrange the equation to solve for wavelength. λ = c ν. Lastly, we plug in our given values and solve. λ = 3.00 × 10 8 m s 1.5 × 10 14 1 s = 2.00 × 10 − 6 m.

  8. Speed of Light Calculator

    The final step is to calculate the total distance that the light has traveled within the time. You can calculate this answer using the speed of light formula: distance = speed of light × time. Thus, the distance that the light can travel in 100 seconds is 299,792,458 m/s × 100 seconds = 29,979,245,800 m. FAQs.

  9. Does Wood Conduct Electricity? (No. But Why?)

    Light, free atomic particles called electrons can freely move within and around atoms. ... The wood will allow a lightning current to travel to the ground's surface. The strike will have an impact on anyone on the ground. Wood sap boils along the path of the strike when lightning strikes. Steam is subsequently created and cells explode in the ...

  10. The Surprising Possibilities of See-Through Wood

    As the thickness gets closer to a centimeter, light transmittance drops: Berglund's group reported that 3.7-millimeter-thick wood—roughly two pennies thick—transmitted only 40 percent of light.

  11. Researchers shine a light through transparent wood

    The researchers report that their three-inch (7.6 cm) block of wood had high transparency and also high haze, which pertains to its ability to scatter light. This kind of material could be ...

  12. Light Year Calculator

    The final step is to calculate the total distance that the light has traveled within the time. You can calculate this answer using the speed of light formula: distance = speed of light × time. Thus, the distance that the light can travel in 100 seconds is 9.46×10¹² km/year × 2 years = 1.892×10¹³ km.

  13. Luminescent wood can light up a room

    Luminescent wood can light up a room Researchers have demonstrated the potential of luminescent wood by using it to light up the interior of a toy house Adapted from ACS Nano 2020, DOI: 10.1021 ...

  14. Ask Ellen: How does light travel from the sun to Earth?

    In fact, light is the fastest thing known to man. It travels at more than 183,292 miles per second! Even at that fast speed, it takes photons of light a full 8 minutes and 30 seconds to travel ...

  15. Light Seems to Pass through Solid Metal

    While some of the light from the flashlight will travel through its holes, the solid part of the colander will keep much of the light from shining through. ... such as clothing, plastics and wood ...

  16. Can Light Travel Through Wood

    Yes, light can travel through wood. However, the amount of light that travels through the wood depends on the type of wood and the thickness of the wood. Oak and other hardwoods allow for more light to pass through than softwoods. And, the thicker the wood, the less light will pass through. So, if you are looking to use wood as a light blocker ...

  17. What makes glass transparent?

    For example, ultraviolet light, which has a wavelength ranging from 10 to 400 nanometers, can't pass through most oxide glasses, such as the glass in a window pane. This makes a window, including the window in our hypothetical house under construction, as opaque to ultraviolet light as wood is to visible light.

  18. How do signals go through solid objects?

    How can wifi penetrate through walls when visible light can't? 0. Can light travel through solid substances such as wood? 1. How is the energy carried by light exchanged with the solid? 73. How strong are Wi-Fi signals? 2.

  19. Structural changes in wood under artificial UV light irradiation

    Use of UV absorbers (UVA) and hindered amine light stabilizers (HALS) both in the coating system and also in a direct wood impregnation step can improve the light stability considerably (Schaller and Rogez 2007; Forsthuber and Grüll 2010). Usually, an efficient protection strategy is to impreg-nate the wood surface with a solution of HALS ...

  20. 35 Incredible Lighthouses You Can Visit In The US

    Boston Light on Little Brewster island has probably the most eventful history of any in the country. The first structure, made of rubblestone and wood, was built in 1716 and was lit by candles.

  21. Why light cannot pass through wall whereas Sound can?

    Can light travel through solid substances such as wood? 1. Understanding the color of fluorescent and non-fluorescent objects. 4. How does electric energy travel at speed of light when electron drift speed is so slow it cannot dissipate the voltage difference in that time? 0.

  22. ANGRY ORCHARD IS HELPING SPRING BREAKERS TRAVEL LIGHT TO

    WALDEN, N.Y., Feb. 29, 2024 (GLOBE NEWSWIRE) -- BYOB is a surefire way to save time and money while spring breaking, but jamming cans and bottles into back seats or travel bags on the way to your ...