electromagnetic waves travel in air

Anatomy of an Electromagnetic Wave

Energy, a measure of the ability to do work, comes in many forms and can transform from one type to another. Examples of stored or potential energy include batteries and water behind a dam. Objects in motion are examples of kinetic energy. Charged particles—such as electrons and protons—create electromagnetic fields when they move, and these fields transport the type of energy we call electromagnetic radiation, or light.

What are Electromagnetic and Mechanical waves?

Mechanical waves and electromagnetic waves are two important ways that energy is transported in the world around us. Waves in water and sound waves in air are two examples of mechanical waves. Mechanical waves are caused by a disturbance or vibration in matter, whether solid, gas, liquid, or plasma. Matter that waves are traveling through is called a medium. Water waves are formed by vibrations in a liquid and sound waves are formed by vibrations in a gas (air). These mechanical waves travel through a medium by causing the molecules to bump into each other, like falling dominoes transferring energy from one to the next. Sound waves cannot travel in the vacuum of space because there is no medium to transmit these mechanical waves.

ELECTROMAGNETIC WAVES

Electricity can be static, like the energy that can make your hair stand on end. Magnetism can also be static, as it is in a refrigerator magnet. A changing magnetic field will induce a changing electric field and vice-versa—the two are linked. These changing fields form electromagnetic waves. Electromagnetic waves differ from mechanical waves in that they do not require a medium to propagate. This means that electromagnetic waves can travel not only through air and solid materials, but also through the vacuum of space.

In the 1860's and 1870's, a Scottish scientist named James Clerk Maxwell developed a scientific theory to explain electromagnetic waves. He noticed that electrical fields and magnetic fields can couple together to form electromagnetic waves. He summarized this relationship between electricity and magnetism into what are now referred to as "Maxwell's Equations."

Heinrich Hertz, a German physicist, applied Maxwell's theories to the production and reception of radio waves. The unit of frequency of a radio wave -- one cycle per second -- is named the hertz, in honor of Heinrich Hertz.

His experiment with radio waves solved two problems. First, he had demonstrated in the concrete, what Maxwell had only theorized — that the velocity of radio waves was equal to the velocity of light! This proved that radio waves were a form of light! Second, Hertz found out how to make the electric and magnetic fields detach themselves from wires and go free as Maxwell's waves — electromagnetic waves.

WAVES OR PARTICLES? YES!

Light is made of discrete packets of energy called photons. Photons carry momentum, have no mass, and travel at the speed of light. All light has both particle-like and wave-like properties. How an instrument is designed to sense the light influences which of these properties are observed. An instrument that diffracts light into a spectrum for analysis is an example of observing the wave-like property of light. The particle-like nature of light is observed by detectors used in digital cameras—individual photons liberate electrons that are used for the detection and storage of the image data.

POLARIZATION

One of the physical properties of light is that it can be polarized. Polarization is a measurement of the electromagnetic field's alignment. In the figure above, the electric field (in red) is vertically polarized. Think of a throwing a Frisbee at a picket fence. In one orientation it will pass through, in another it will be rejected. This is similar to how sunglasses are able to eliminate glare by absorbing the polarized portion of the light.

DESCRIBING ELECTROMAGNETIC ENERGY

The terms light, electromagnetic waves, and radiation all refer to the same physical phenomenon: electromagnetic energy. This energy can be described by frequency, wavelength, or energy. All three are related mathematically such that if you know one, you can calculate the other two. Radio and microwaves are usually described in terms of frequency (Hertz), infrared and visible light in terms of wavelength (meters), and x-rays and gamma rays in terms of energy (electron volts). This is a scientific convention that allows the convenient use of units that have numbers that are neither too large nor too small.

The number of crests that pass a given point within one second is described as the frequency of the wave. One wave—or cycle—per second is called a Hertz (Hz), after Heinrich Hertz who established the existence of radio waves. A wave with two cycles that pass a point in one second has a frequency of 2 Hz.

Electromagnetic waves have crests and troughs similar to those of ocean waves. The distance between crests is the wavelength. The shortest wavelengths are just fractions of the size of an atom, while the longest wavelengths scientists currently study can be larger than the diameter of our planet!

An electromagnetic wave can also be described in terms of its energy—in units of measure called electron volts (eV). An electron volt is the amount of kinetic energy needed to move an electron through one volt potential. Moving along the spectrum from long to short wavelengths, energy increases as the wavelength shortens. Consider a jump rope with its ends being pulled up and down. More energy is needed to make the rope have more waves.

Next: Wave Behaviors

National Aeronautics and Space Administration, Science Mission Directorate. (2010). Anatomy of an Electromagnetic Wave. Retrieved [insert date - e.g. August 10, 2016] , from NASA Science website: http://science.nasa.gov/ems/02_anatomy

Science Mission Directorate. "Anatomy of an Electromagnetic Wave" NASA Science . 2010. National Aeronautics and Space Administration. [insert date - e.g. 10 Aug. 2016] http://science.nasa.gov/ems/02_anatomy

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13.1: Electromagnetic Waves

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Did you ever wonder how a microwave works? It directs invisible waves of radiation toward the food placed inside of it. The radiation transfers energy to the food, causing it to get warmer. The radiation is in the form of microwaves, which are a type of electromagnetic waves.

What Are Electromagnetic Waves?

Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. Like other waves, electromagnetic waves transfer energy from one place to another. The transfer of energy by electromagnetic waves is called electromagnetic radiation . Electromagnetic waves can transfer energy through matter or across empty space.

Q: How do microwaves transfer energy inside a microwave oven?

A: They transfer energy through the air inside the oven to the food.

May the Force Be With You

A familiar example may help you understand the vibrating electric and magnetic fields that make up electromagnetic waves. Consider a bar magnet, like the one in the Figure below. The magnet exerts magnetic force over an area all around it. This area is called a magnetic field. The field lines in the diagram represent the direction and location of the magnetic force. Because of the field surrounding a magnet, it can exert force on objects without touching them. They just have to be within its magnetic field.

Q: How could you demonstrate that a magnet can exert force on objects without touching them?

A: You could put small objects containing iron, such as paper clips, near a magnet and show that they move toward the magnet.

An electric field is similar to a magnetic field. It is an area of electrical force surrounding a positively or negatively charged particle. You can see electric fields in the following Figure below. Like a magnetic field, an electric field can exert force on objects over a distance without actually touching them.

How an Electromagnetic Wave Begins

An electromagnetic wave begins when an electrically charged particle vibrates. The Figure below shows how this happens. A vibrating charged particle causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field. The two types of vibrating fields combine to create an electromagnetic wave.

How an Electromagnetic Wave Travels

As you can see in the Figure above, the electric and magnetic fields that make up an electromagnetic wave are perpendicular (at right angles) to each other. Both fields are also perpendicular to the direction that the wave travels. Therefore, an electromagnetic wave is a transverse wave. However, unlike a mechanical transverse wave, which can only travel through matter, an electromagnetic transverse wave can travel through empty space. When waves travel through matter, they lose some energy to the matter as they pass through it. But when waves travel through space, no energy is lost. Therefore, electromagnetic waves don’t get weaker as they travel. However, the energy is “diluted” as it travels farther from its source because it spreads out over an ever-larger area.

Electromagnetic Wave Interactions

When electromagnetic waves strike matter, they may interact with it in the same ways that mechanical waves interact with matter. Electromagnetic waves may:

• reflect, or bounce back from a surface;
• refract, or bend when entering a new medium;
• diffract, or spread out around obstacles.

Electromagnetic waves may also be absorbed by matter and converted to other forms of energy. Microwaves are a familiar example. When microwaves strike food in a microwave oven, they are absorbed and converted to thermal energy, which heats the food.

Sources of Electromagnetic Waves

The most important source of electromagnetic waves on Earth is the sun. Electromagnetic waves travel from the sun to Earth across space and provide virtually all the energy that supports life on our planet. Many other sources of electromagnetic waves depend on technology. Radio waves, microwaves, and X rays are examples. We use these electromagnetic waves for communications, cooking, medicine, and many other purposes.

Launch the Light Wave simulation below to help you visualize light as a transverse wave moving through the electromagnetic field. Be sure to adjust the wavelength band slider to observe waves of different sizes, such as Radio Waves and X-Rays. There is also an illustration of objects of comparable sizes next to the electromagnetic spectrum to help you imagine the sizes of these invisible light waves.

Interactive Element

• Electromagnetic waves are waves that consist of vibrating electric and magnetic fields. They transfer energy through matter or across space. The transfer of energy by electromagnetic waves is called electromagnetic radiation.
• The electric and magnetic fields of an electromagnetic wave are areas of electric or magnetic force. The fields can exert force over objects at a distance.
• An electromagnetic wave begins when an electrically charged particle vibrates. This causes a vibrating electric field, which in turn creates a vibrating magnetic field. The two vibrating fields together form an electromagnetic wave.
• An electromagnetic wave is a transverse wave that can travel across space as well as through matter. When it travels through space, it doesn’t lose energy to a medium as a mechanical wave does.
• When electromagnetic waves strike matter, they may be reflected, refracted, or diffracted. Or they may be absorbed by matter and converted to other forms of energy.
• The most important source of electromagnetic waves on Earth is the sun. Many other sources of electromagnetic waves depend on technology.
• What is an electromagnetic wave?
• Define electromagnetic radiation.
• Describe the electric and magnetic fields of an electromagnetic wave.
• How does an electromagnetic wave begin? How does it travel?
• Compare and contrast electromagnetic and mechanical transverse waves.
• List three sources of electromagnetic waves on Earth.

Explore More

Watch the electromagnetic wave animation and then answer the questions below.

• Identify the vibrating electric and magnetic fields of the wave.
• Describe the direction in which the wave is traveling.

Additional Resources

Study Guide: Wave Optics Study Guide

Real World Application: Printing with Light, The Incredible Hulk

PLIX: Play, Learn, Interact, eXplore: Energy Levels: Bohr's Atomic Model

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Middle school physics - NGSS

Course: middle school physics - ngss   >   unit 4, mechanical waves and light.

• Understand: mechanical waves and light

Key points:

• Sound waves, water waves, and seismic waves are all types of mechanical waves.
• Light is a form of electromagnetic wave.
• A light wave’s amplitude determines how intense, or bright, it is. Its frequency determines the light wave’s color.
• A sound wave’s amplitude determines how loud it is. Its frequency determines the sound wave’s pitch.

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Electromagnetic waves are a form of radiation that travel though the universe. They are formed when an electric field (Fig. 1 red arrows) couples with a magnetic field (Fig.1 blue arrows).

Both electricity and magnetism can be static (respectively, what holds a balloon to the wall or a refrigerator magnet to metal), but when they change or move together, they make waves. Magnetic and electric fields of an electromagnetic wave are perpendicular to each other and to the direction of the wave.

Unlike sound waves, which must travel through matter by bumping molecules into each other like dominoes (and thus can not travel through a vacuum like space), electromagnetic waves do not need molecules to travel. They can travel through air, solid objects, and even space, making them very useful for a lot of technologies.

When you listen to the radio, connect to a wireless network, or cook dinner in a microwave oven, you are using electromagnetic waves. Radio waves and microwaves are two types of electromagnetic waves. They only differ from each other in wavelength – the distance between one wave crest to the next.

While most of this energy is invisible to us, we can see the range of wavelengths that we call light. This visible part of the electromagnetic spectrum consists of the colors that we see in a rainbow – red, orange, yellow, green, blue, indigo, and violet. Each of these colors also corresponds to a different measurable wavelength of light.

Waves in the electromagnetic spectrum vary in size from very long radio waves that are the length of buildings to very short gamma-rays that are smaller than the nucleus of an atom.

Their size is related to their energy. The smaller the wavelength, the higher the energy. For example, a brick wall blocks the relatively larger and lower-energy wavelengths of visible light but not the smaller, more energetic x-rays. A denser material such as lead, however, can block x-rays.

While it’s commonly said that waves are "blocked" by certain materials, the correct understanding is that wavelengths of energy are absorbed by the material. This understanding is critical to interpreting data from weather satellites because the atmosphere also absorbs some wavelengths while allowing others to pass through.

16.2 Plane Electromagnetic Waves

Learning objectives.

By the end of this section, you will be able to:

• Describe how Maxwell’s equations predict the relative directions of the electric fields and magnetic fields, and the direction of propagation of plane electromagnetic waves
• Explain how Maxwell’s equations predict that the speed of propagation of electromagnetic waves in free space is exactly the speed of light
• Calculate the relative magnitude of the electric and magnetic fields in an electromagnetic plane wave
• Describe how electromagnetic waves are produced and detected

Mechanical waves travel through a medium such as a string, water, or air. Perhaps the most significant prediction of Maxwell’s equations is the existence of combined electric and magnetic (or electromagnetic) fields that propagate through space as electromagnetic waves. Because Maxwell’s equations hold in free space, the predicted electromagnetic waves, unlike mechanical waves, do not require a medium for their propagation.

A general treatment of the physics of electromagnetic waves is beyond the scope of this textbook. We can, however, investigate the special case of an electromagnetic wave that propagates through free space along the x -axis of a given coordinate system.

Electromagnetic Waves in One Direction

An electromagnetic wave consists of an electric field, defined as usual in terms of the force per charge on a stationary charge, and a magnetic field, defined in terms of the force per charge on a moving charge. The electromagnetic field is assumed to be a function of only the x -coordinate and time. The y -component of the electric field is then written as E y ( x , t ) , E y ( x , t ) , the z -component of the magnetic field as B z ( x , t ) B z ( x , t ) , etc. Because we are assuming free space, there are no free charges or currents, so we can set Q in = 0 Q in = 0 and I = 0 I = 0 in Maxwell’s equations.

The transverse nature of electromagnetic waves

We examine first what Gauss’s law for electric fields implies about the relative directions of the electric field and the propagation direction in an electromagnetic wave. Assume the Gaussian surface to be the surface of a rectangular box whose cross-section is a square of side l and whose third side has length Δ x Δ x , as shown in Figure 16.6 . Because the electric field is a function only of x and t , the y -component of the electric field is the same on both the top (labeled Side 2) and bottom (labeled Side 1) of the box, so that these two contributions to the flux cancel. The corresponding argument also holds for the net flux from the z -component of the electric field through Sides 3 and 4. Any net flux through the surface therefore comes entirely from the x -component of the electric field. Because the electric field has no y - or z -dependence, E x ( x , t ) E x ( x , t ) is constant over the face of the box with area A and has a possibly different value E x ( x + Δ x , t ) E x ( x + Δ x , t ) that is constant over the opposite face of the box. Applying Gauss’s law gives

where A = l × l A = l × l is the area of the front and back faces of the rectangular surface. But the charge enclosed is Q in = 0 Q in = 0 , so this component’s net flux is also zero, and Equation 16.13 implies E x ( x , t ) = E x ( x + Δ x , t ) E x ( x , t ) = E x ( x + Δ x , t ) for any Δ x Δ x . Therefore, if there is an x -component of the electric field, it cannot vary with x . A uniform field of that kind would merely be superposed artificially on the traveling wave, for example, by having a pair of parallel-charged plates. Such a component E x ( x , t ) E x ( x , t ) would not be part of an electromagnetic wave propagating along the x -axis; so E x ( x , t ) = 0 E x ( x , t ) = 0 for this wave. Therefore, the only nonzero components of the electric field are E y ( x , t ) E y ( x , t ) and E z ( x , t ) , E z ( x , t ) , perpendicular to the direction of propagation of the wave.

A similar argument holds by substituting E for B and using Gauss’s law for magnetism instead of Gauss’s law for electric fields. This shows that the B field is also perpendicular to the direction of propagation of the wave. The electromagnetic wave is therefore a transverse wave, with its oscillating electric and magnetic fields perpendicular to its direction of propagation.

The speed of propagation of electromagnetic waves

We can next apply Maxwell’s equations to the description given in connection with Figure 16.4 in the previous section to obtain an equation for the E field from the changing B field, and for the B field from a changing E field. We then combine the two equations to show how the changing E and B fields propagate through space at a speed precisely equal to the speed of light.

First, we apply Faraday’s law over Side 3 of the Gaussian surface, using the path shown in Figure 16.7 . Because E x ( x , t ) = 0 , E x ( x , t ) = 0 , we have

Assuming Δ x Δ x is small and approximating E y ( x + Δ x , t ) E y ( x + Δ x , t ) by

Because Δ x Δ x is small, the magnetic flux through the face can be approximated by its value in the center of the area traversed, namely B z ( x + Δ x 2 , t ) B z ( x + Δ x 2 , t ) . The flux of the B field through Face 3 is then the B field times the area,

From Faraday’s law,

Therefore, from Equation 16.13 and Equation 16.14 ,

Canceling l Δ x l Δ x and taking the limit as Δ x = 0 Δ x = 0 , we are left with

We could have applied Faraday’s law instead to the top surface (numbered 2) in Figure 16.7 , to obtain the resulting equation

This is the equation describing the spatially dependent E field produced by the time-dependent B field.

Next we apply the Ampère-Maxwell law (with I = 0 I = 0 ) over the same two faces (Surface 3 and then Surface 2) of the rectangular box of Figure 16.7 . Applying Equation 16.10 ,

to Surface 3, and then to Surface 2, yields the two equations

These equations describe the spatially dependent B field produced by the time-dependent E field.

We next combine the equations showing the changing B field producing an E field with the equation showing the changing E field producing a B field. Taking the derivative of Equation 16.16 with respect to x and using Equation 16.19 gives

This is the form taken by the general wave equation for our plane wave. Because the equations describe a wave traveling at some as-yet-unspecified speed c , we can assume the field components are each functions of x – ct for the wave traveling in the + x -direction, that is,

It is left as a mathematical exercise to show, using the chain rule for differentiation, that Equation 16.17 and Equation 16.18 imply

The speed of the electromagnetic wave in free space is therefore given in terms of the permeability and the permittivity of free space by

We could just as easily have assumed an electromagnetic wave with field components E z ( x , t ) E z ( x , t ) and B y ( x , t ) B y ( x , t ) . The same type of analysis with Equation 16.25 and Equation 16.24 would also show that the speed of an electromagnetic wave is c = 1 / ε 0 μ 0 c = 1 / ε 0 μ 0 .

The physics of traveling electromagnetic fields was worked out by Maxwell in 1873. He showed in a more general way than our derivation that electromagnetic waves always travel in free space with a speed given by Equation 16.18 . If we evaluate the speed c = 1 ε 0 μ 0 , c = 1 ε 0 μ 0 , we find that

which is the speed of light . Imagine the excitement that Maxwell must have felt when he discovered this equation! He had found a fundamental connection between two seemingly unrelated phenomena: electromagnetic fields and light.

Check Your Understanding 16.3

The wave equation was obtained by (1) finding the E field produced by the changing B field, (2) finding the B field produced by the changing E field, and combining the two results. Which of Maxwell’s equations was the basis of step (1) and which of step (2)?

How the E and B Fields Are Related

So far, we have seen that the rates of change of different components of the E and B fields are related, that the electromagnetic wave is transverse, and that the wave propagates at speed c . We next show what Maxwell’s equations imply about the ratio of the E and B field magnitudes and the relative directions of the E and B fields.

We now consider solutions to Equation 16.16 in the form of plane waves for the electric field:

We have arbitrarily taken the wave to be traveling in the +x -direction and chosen its phase so that the maximum field strength occurs at the origin at time t = 0 t = 0 . We are justified in considering only sines and cosines in this way, and generalizing the results, because Fourier’s theorem implies we can express any wave, including even square step functions, as a superposition of sines and cosines.

At any one specific point in space, the E field oscillates sinusoidally at angular frequency ω ω between + E 0 + E 0 and − E 0 , − E 0 , and similarly, the B field oscillates between + B 0 + B 0 and − B 0 . − B 0 . The amplitude of the wave is the maximum value of E y ( x , t ) . E y ( x , t ) . The period of oscillation T is the time required for a complete oscillation. The frequency f is the number of complete oscillations per unit of time, and is related to the angular frequency ω ω by ω = 2 π f ω = 2 π f . The wavelength λ λ is the distance covered by one complete cycle of the wave, and the wavenumber k is the number of wavelengths that fit into a distance of 2 π 2 π in the units being used. These quantities are related in the same way as for a mechanical wave:

Given that the solution of E y E y has the form shown in Equation 16.20 , we need to determine the B field that accompanies it. From Equation 16.24 , the magnetic field component B z B z must obey

Because the solution for the B -field pattern of the wave propagates in the + x -direction at the same speed c as the E- field pattern, it must be a function of k ( x − c t ) = k x − ω t k ( x − c t ) = k x − ω t . Thus, we conclude from Equation 16.21 that B z B z is

These results may be written as

Therefore, the peaks of the E and B fields coincide, as do the troughs of the wave, and at each point, the E and B fields are in the same ratio equal to the speed of light c . The plane wave has the form shown in Figure 16.8 .

Example 16.2

Calculating b -field strength in an electromagnetic wave, significance.

Changing electric fields create relatively weak magnetic fields. The combined electric and magnetic fields can be detected in electromagnetic waves, however, by taking advantage of the phenomenon of resonance, as Hertz did. A system with the same natural frequency as the electromagnetic wave can be made to oscillate. All radio and TV receivers use this principle to pick up and then amplify weak electromagnetic waves, while rejecting all others not at their resonant frequency.

Check Your Understanding 16.4

What conclusions did our analysis of Maxwell’s equations lead to about these properties of a plane electromagnetic wave: (a) the relative directions of wave propagation, of the E field, and of B field, (b) the speed of travel of the wave and how the speed depends on frequency, and (c) the relative magnitudes of the E and B fields.

Production and Detection of Electromagnetic Waves

A steady electric current produces a magnetic field that is constant in time and which does not propagate as a wave. Accelerating charges, however, produce electromagnetic waves. An electric charge oscillating up and down, or an alternating current or flow of charge in a conductor, emit radiation at the frequencies of their oscillations. The electromagnetic field of a dipole antenna is shown in Figure 16.9 . The positive and negative charges on the two conductors are made to reverse at the desired frequency by the output of a transmitter as the power source. The continually changing current accelerates charge in the antenna, and this results in an oscillating electric field a distance away from the antenna. The changing electric fields produce changing magnetic fields that in turn produce changing electric fields, which thereby propagate as electromagnetic waves. The frequency of this radiation is the same as the frequency of the ac source that is accelerating the electrons in the antenna. The two conducting elements of the dipole antenna are commonly straight wires. The total length of the two wires is typically about one-half of the desired wavelength (hence, the alternative name half-wave antenna ), because this allows standing waves to be set up and enhances the effectiveness of the radiation.

The electric field lines in one plane are shown. The magnetic field is perpendicular to this plane. This radiation field has cylindrical symmetry around the axis of the dipole. Field lines near the dipole are not shown. The pattern is not at all uniform in all directions. The strongest signal is in directions perpendicular to the axis of the antenna, which would be horizontal if the antenna is mounted vertically. There is zero intensity along the axis of the antenna. The fields detected far from the antenna are from the changing electric and magnetic fields inducing each other and traveling as electromagnetic waves. Far from the antenna, the wave fronts, or surfaces of equal phase for the electromagnetic wave, are almost spherical. Even farther from the antenna, the radiation propagates like electromagnetic plane waves.

The electromagnetic waves carry energy away from their source, similar to a sound wave carrying energy away from a standing wave on a guitar string. An antenna for receiving electromagnetic signals works in reverse. Incoming electromagnetic waves induce oscillating currents in the antenna, each at its own frequency. The radio receiver includes a tuner circuit, whose resonant frequency can be adjusted. The tuner responds strongly to the desired frequency but not others, allowing the user to tune to the desired broadcast. Electrical components amplify the signal formed by the moving electrons. The signal is then converted into an audio and/or video format.

Interactive

Use this simulation to broadcast radio waves. Wiggle the transmitter electron manually or have it oscillate automatically. Display the field as a curve or vectors. The strip chart shows the electron positions at the transmitter and at the receiver.

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Propagation of an Electromagnetic Wave

Electromagnetic waves are waves which can travel through the vacuum of outer space. Mechanical waves, unlike electromagnetic waves, require the presence of a material medium in order to transport their energy from one location to another. Sound waves are examples of mechanical waves while light waves are examples of electromagnetic waves.

Electromagnetic waves are created by the vibration of an electric charge. This vibration creates a wave which has both an electric and a magnetic component. An electromagnetic wave transports its energy through a vacuum at a speed of 3.00 x 10 8 m/s (a speed value commonly represented by the symbol c ). The propagation of an electromagnetic wave through a material medium occurs at a net speed which is less than 3.00 x 10 8 m/s. This is depicted in the animation below.

The mechanism of energy transport through a medium involves the absorption and reemission of the wave energy by the atoms of the material. When an electromagnetic wave impinges upon the atoms of a material, the energy of that wave is absorbed. The absorption of energy causes the electrons within the atoms to undergo vibrations. After a short period of vibrational motion, the vibrating electrons create a new electromagnetic wave with the same frequency as the first electromagnetic wave. While these vibrations occur for only a very short time, they delay the motion of the wave through the medium. Once the energy of the electromagnetic wave is reemitted by an atom, it travels through a small region of space between atoms. Once it reaches the next atom, the electromagnetic wave is absorbed, transformed into electron vibrations and then reemitted as an electromagnetic wave. While the electromagnetic wave will travel at a speed of c (3 x 10 8 m/s) through the vacuum of interatomic space, the absorption and reemission process causes the net speed of the electromagnetic wave to be less than c. This is observed in the animation below.

The actual speed of an electromagnetic wave through a material medium is dependent upon the optical density of that medium. Different materials cause a different amount of delay due to the absorption and reemission process. Furthermore, different materials have their atoms more closely packed and thus the amount of distance between atoms is less. These two factors are dependent upon the nature of the material through which the electromagnetic wave is traveling. As a result, the speed of an electromagnetic wave is dependent upon the material through which it is traveling.

For more information on physical descriptions of waves, visit The Physics Classroom Tutorial . Detailed information is available there on the following topics:

Mechanical vs. Electromagnetic Waves Wavelike Behaviors of Light The EM and Visible Spectra Light Absorption, Reflection, and Transmission Optical Density and Light Speed  

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How do electromagnetic waves propagate through different media?

Electromagnetic waves, such as light, radio waves, and microwaves, are an essential aspect of our daily lives. These waves propagate through various media, including vacuum, air, and solid materials.

How Electromagnetic Waves Propagate Through Different Media

Electromagnetic waves, such as light, radio waves, and microwaves, are an essential aspect of our daily lives. These waves propagate through various media, including vacuum, air, and solid materials. In this article, we will explore how electromagnetic waves travel through different media and how their speed and behavior change as they pass through these materials.

Propagation in Vacuum

Electromagnetic waves propagate most efficiently through a vacuum, traveling at the speed of light, approximately 299,792 kilometers per second (km/s). In a vacuum, there are no particles or obstacles to impede their path, allowing them to maintain their original speed and direction.

Propagation in Air

As electromagnetic waves pass through the Earth’s atmosphere, they encounter air molecules, which can cause them to scatter, absorb, or refract. Despite these interactions, their propagation speed remains close to that in a vacuum. The index of refraction for air is about 1.0003, which means the speed of light in air is roughly 299,702 km/s, just slightly slower than in a vacuum.

Propagation in Transparent Media

When electromagnetic waves enter a transparent medium, such as water or glass, their speed changes due to the medium’s index of refraction. This change in speed causes the waves to bend, a phenomenon known as refraction. The index of refraction for a given material is defined as the ratio of the speed of light in a vacuum to the speed of light in the material.

• Water: Index of refraction ≈ 1.33, Speed ≈ 225,056 km/s
• Glass: Index of refraction ≈ 1.5, Speed ≈ 199,861 km/s

As the index of refraction increases, the speed of light within the material decreases, leading to more significant refraction.

Propagation in Conductive Media

Electromagnetic waves propagating through conductive materials, such as metals, interact with the free electrons in the material. This interaction causes the waves to be absorbed and converted into heat, significantly reducing their propagation speed and depth. Radio waves, for example, can penetrate only a few millimeters into a conductive material before being absorbed.

Propagation in Lossy Dielectric Media

Some materials, like soil or human tissue, are considered lossy dielectric media. These materials have a dielectric constant and a loss tangent that determines the degree to which electromagnetic waves are absorbed and attenuated. In these materials, the propagation of waves is significantly affected, with both attenuation and absorption playing a role in the decrease of wave strength as they travel through the medium.

In conclusion, electromagnetic waves’ propagation through different media is influenced by factors such as the medium’s index of refraction, dielectric constant, and loss tangent. Understanding how electromagnetic waves interact with various media is crucial in fields like telecommunications, optics, and medical imaging.

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Springer Handbook of Ocean Engineering pp 177–196 Cite as

Ocean Electromagnetics

• John J. Holmes 3  

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Even though acoustic waves can travel long distances in the sea with little attenuation, ocean electromagnetics has important applications in the areas of geophysical surveys and searches of the seafloor and sub-bottom, communications across the sea–air boundary, and high data transfer rate at short ranges. Unlike in-air propagation of electromagnetic fields, the finite conductivity of seawater results in a frequency-dependent phase velocity, attenuation, intrinsic impedance, and reflection and transmission coefficients at the ocean’s surface. After giving a short summary of the electric and magnetic properties of the ocean, this chapter begins with Maxwell’s equations and develops the mathematical descriptions of electromagnetic fields and dipole sources within a conducting media. The differences between plane wave reflection and transmission at the surface of fresh water and seawater are used to highlight how electromagnetic propagation within the electrically conducting ocean is so very different than the more familiar radio frequency transmissions in air. In addition, equations are presented that describe the fields from submerged electric and magnetic dipoles that are located both far and near the sea surface. These formulations are valid over the frequency range from 0 Hz to a few MHz. Finally, a brief discussion of ocean electromagnetics at optical wavelengths is given at the end of this chapter.

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Abbreviations

apparent optical properties

extremely low frequency

horizontal electric dipole

horizontal magnetic dipole

ultralow frequency

vertical electric dipole

vertical magnetic dipole

J.R. Apel: Principles of Ocean Physics (Academic Press, San Diego 1987)

Google Scholar  

C.A. Balanis: Advanced Engineering Electromagnetics , 2nd edn. (Wiley, Hoboken 2012)

C.A. Balanis: Antenna Theory , 3rd edn. (Wiley, Hoboken 2005) pp. 133–142

J. Mosig: The weighted averages algorithm revisited, IEEE Trans. Antennas Propag. 60 (4), 2011–2018 (2012)

Article   MathSciNet   Google Scholar  

J.R. Wait: Electromagnetic Fields of Sources in Lossy Media. In: Antenna Theory , ed. by R.E. Collin, F.J. Zucker (McGraw-Hill, New York 1969) pp. 476–478

A.C. Fraser-Smith, D.M. Bubenik: ULF/ELF electromagnetic fields generated above a sea of finite depth by a submerged vertically-directed harmonic magnetic dipole, Radio Sci. 14 , 59–74 (1979)

Article   Google Scholar  

D.M. Bubenik, A.C. Fraser-Smith: ULF/ELF electromagnetic fields generated in a sea of finite depth by a submerged vertically-directed harmonic magnetic dipole, Radio Sci. 13 , 1011–1020 (1978)

J.R. Wait: Electromagnetic Waves in Stratified Media (IEEE Press, Piscataway 1996) pp. 143–146

C. Liu, L.G. Zheng, Y.P. Li: Study of ELF electromagnetic fields from a submerged horizontal electric dipole positioned in a sea of finite depth, IEEE 3rd Int. Symp. Microw. Antenna Propag. EMC Technol. Wirel. Commun. (2009) pp. 152–157

A.C. Fraser-Smith, D.M. Bubenik: Compendium of the ULF/ELF Electromagnetic Fields Generated Above a Sea of Finite Depth by Submerged Harmonic Dipoles , Tech. Rep., Vol. E715-1 (Stanford Univ., Stanford 1980)

A.C. Fraser-Smith, D.M. Bubenik: ULF/ELF/VLF Electromagnetic Fields Generated in a Sea of Finite Depth by Elevated Dipole Sources , Tech. Rep., Vol. E715-2 (Stanford Univ., Stanford 1984)

D.L. Jones, C.P. Burke: The DC field components of horizontal and vertical electric dipole sources immersed in three-layer stratified media, Ann. Geophys. 15 , 503–510 (1997)

M.B. Kraichman: Electromagnetic propagation in conducting media. In: Electromagnetics Problem Solver , ed. by M. Fogiel (REA, Piscataway 2000) pp. 3–24, Section II

P.R. Bannister, R.L. Dube: Simple expressions for horizontal electric dipole quasi-static range subsurface-to-subsurface and subsurface-to-air propagation, Radio Sci. 13 (3), 501–507 (1978)

W.C. Cox: A 1 Mbps Underwater Communication System Using a 405 nm Laser Diode and Photomultiplier Tube, Master Thesis (North Carolina State Univ., Raleigh 2007)

R.C. Smith, K.S. Baker: Optical properties of the clearest natural waters (200–800 nm), Appl. Opt. 20 (2), 177–184 (1971)

R.W. Austin, T.J. Petzold: Spectral dependence of the diffuse attenuation coefficient of light in ocean waters, Opt. Engr. 25 , 471 (1986)

C.J. Cassidy: Airborne Laser Mine Detection System, Master Thesis (Naval Postgraduate School, Monterey 1995)

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Underwater Electromagnetic Signatures and Technology Division, Naval Surface Warfare Center, 9500 MacArthur Boulevard, MD 20817-5700, West Bethesda, Maryland, USA

John J. Holmes

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The Inst. for Ocean and Systems Engineering – SeaTech, Florida Atlantic University, 101 North Beach Road, FL 33004, Dania Beach, Florida, USA

Manhar R. Dhanak

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Nikolaos I. Xiros

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Holmes, J.J. (2016). Ocean Electromagnetics. In: Dhanak, M.R., Xiros, N.I. (eds) Springer Handbook of Ocean Engineering. Springer Handbooks. Springer, Cham. https://doi.org/10.1007/978-3-319-16649-0_8

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5.1.1: Speeds of Different Types of Waves

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• Kyle Forinash and Wolfgang Christian

The speed of a wave is fixed by the type of wave and the physical properties of the medium in which it travels. An exception is electromagnetic waves which can travel through a vacuum. For most substances the material will vibrate obeying a Hooke's law force as a wave passes through it and the speed will not depend on frequency. Electromagnetic waves in a vacuum and waves traveling though a linear medium are termed linear waves and have constant speed. Examples:

• For sound waves in a fluid (for example air or water) the speed is determined by \(v=(B/\rho )^{1/2}\) where \(B\) is the bulk modulus or compressibility of the fluid in newtons per meter squared and \(\rho\) is the density in kilograms per cubic meter.
• For sound waves in a solid the speed is determined by \(v= (Y/\rho )^{1/2}\) where \(Y\) is Young's modulus or stiffness in Newtons per meter squared and \(\rho\) is the density in kilograms per meter cubed.
• For waves on a string the speed is determined by \(v=(T/\mu )^{1/2}\) where \(T\) is the tension in the string in Newtons and \(\mu\) is the mass per length in kilograms per meter.
• Although electromagnetic waves do not need a medium to travel (they can travel through a vacuum) their speed in a vacuum, \(c = (1/\mu _{o} ε_{o})^{1/2} = 3.0\times 10^{8}\text{ m/s}\) is governed by two physical constants, the permeability \(\mu_{o}\) and the permittivity, \(ε_{o}\) of free space (vacuum).

Table \(\PageIndex{1}\)

Here is a more comprehensive list of the speed of sound in various materials .

As we saw in the previous chapter, there is a relationship between the period, wavelength and speed of the wave. The period of a cork floating in the water is affected by how fast the wave passes (wave speed) and the distance between peaks (wavelength). The relationship between speed, period and wavelength of a sine wave is given by \(v=\lambda /T\) where wavelength and period for a sine wave were defined previously. This can also be written as \(v=\lambda f\) since frequency is the inverse of period and is true for all linear waves. Notice that, since wave speed is normally a fixed quantity the frequency and wavelength will be inversely proportion; higher frequencies mean shorter wavelengths.

Often it is easier to write \(ω = 2πf\) where \(\omega\) is the angular frequency in radians per second instead of having to write \(2\pi f\) everywhere. Likewise it is easier to write \(k=2\pi /\lambda \) where \(k\) is the wave number in radians per meter rather than having to write \(2\pi /\lambda\) a lot. (Note that \(k\) is not a spring constant here.) Using these new definitions the speed of a wave can also be written as \(v=f\lambda =\omega /k\).

If the medium is uniform the speed of a wave is fixed and does not change. There are circumstances where the speed of a particular wave does change, however. Notice that the speed of sound in air depends on the density of the air (mass per volume). But the density of air changes with temperature and humidity. So the speed of sound can be different on different days and in different locations. The temperature dependence of the speed of sound in air is given by \(v = 344 + 0.6 (T - 20)\) in meters per second where \(T\) is the temperature in Celsius (\(T\) here is temperature, not period). Notice that at room temperature (\(20^{\circ}\text{C}\)) sound travels at \(344\text{ m/s}\).

The speed of sound can also be affected by the movement of the medium in which it travels. For example, wind can carry sound waves further (i.e. faster) if the sound is traveling in the same direction or it can slow the sound down if the sound is traveling in a direction opposite to the wind direction.

Electromagnetic waves travel at \(\text{c} = 3.0\times 10^{8}\text{ m/s}\) in a vacuum but slow down when they pass through a medium (for example light passing from air to glass). This occurs because the material has a different value for the permittivity and/or permeability due to the interaction of the wave with the atoms of the material. The amount the speed changes is given by the index of refraction \(n=c/v\) where \(c\) is the speed of light in a vacuum and \(v\) is the speed in the medium. The frequency of the wave does not change when it slows down so, since \(v=\lambda f\), the wavelength of electromagnetic waves in a medium must be slightly smaller.

Video/audio examples:

• What is the speed of sound in a vacuum? Buzzer in a bell jar . Why is there no sound when the air is removed from the jar?
• Demonstration of speed of sound in different gasses . Why is there no sound when the air is removed from the jar?
• These two videos demonstrate the Allasonic effect. The speed of sound is different in a liquid with air bubbles because the density is different. As the bubbles burst, the speed of sound changes, causing the frequency of sound waves in the liquid column to change, thus changing the pitch. Example: one , two . What do you hear in each case?
• The Zube Tube is a toy that has a spring inside attached to two plastic cups on either end. Vibrations in the spring travel at different speeds so a sound starting at one end (for example a click when you shake the tube and the spring hits the cup) ends up changing pitch at the other end as the various frequencies arrive. In other words this is a nonlinear system. See if you can figure out from the video which frequencies travel faster, high frequencies or low.

Mini-lab on measuring the speed of sound .

Questions on Wave Speed:

\(f=1/T,\quad v=f\lambda ,\quad v=\omega /k,\quad k=2\pi /\lambda,\quad \omega =2\pi f,\quad y(x,t)=A\cos (kx-\omega t+\phi ),\quad v=\sqrt{B/Q}\)

• Light travels at \(3.0\times 10^{8}\text{ m/s}\) but sound waves travel at about \(344\text{ m/s}\). What is the time delay for light and sound to arrive from a source that is \(10,000\text{ m}\) away (this can be used to get an approximate distance to a thunderstorm)?
• What two mistakes are made in science fiction movies where you see and hear an explosion in space at the same time?
• Consult the table for the speed of sound in various substances. If you have one ear in the water and one ear out while swimming in a lake and a bell is rung that is half way in the water some distance away, which ear hears the sound first?
• At \(20\text{C}\) the speed of sound is \(344\text{ m/s}\). How far does sound travel in \(1\text{ s}\)? How far does sound travel in \(60\text{ s}\)?
• Compare the last two answers with the distance traveled by light which has a speed of \(3.0\times 10^{8}\text{ m/s}\). Why do you see something happen before you hear it?
• The speed of sound in water is \(1482\text{ m/s}\). How far does sound travel under water in \(1\text{ s}\)? How far does sound travel under water in \(60\text{ s}\)?
• What happens to the speed of sound in air as temperature increases?
• Using the equation for the speed of sound at different temperatures, what is the speed of sound on a hot day when the temperature is \(30^{\circ}\text{C}\)? Hint: \(v = 344\text{ m/s} + 0.6 (T - 20)\) where \(T\) is the temperature in Celsius.
• Using the speed of sound at \(30^{\circ}\text{C}\) from the last question, recalculate the distance traveled for the cases in question four.
• Suppose on a cold day the temperature is \(-10^{\circ}\text{C}\: (14^{\circ}\text{F}\)). You are playing in the marching band outside. How long does it take the sound from the band to reach the spectators if they are \(100\text{ m}\) away?
• What is the difference in the speed of sound in air on a hot day (\(40^{\circ}\text{C}\)) and a cold day (\(0^{\circ}\text{C}\))?
• What would an orchestra sound like if different instruments produced sounds that traveled at different speeds?
• The speed of a wave is fixed by the medium it travels in so, for a given situation, is usually constant. What happens to the frequency of a wave if the wavelength is doubled?
• What happens to the wavelength of a wave if the frequency is doubled and has the same speed?
• Suppose a sound wave has a frequency \(200\text{ Hz}\). If the speed of sound is \(343\text{ m/s}\), what wavelength is this wave?
• What factors determine the speed of sound in air?
• Why do sound waves travel faster through liquids than air?
• Why do sound waves travel faster through solids than liquids?
• The speed of sound in a fluid is given by \(v=\sqrt{B/Q}\) where \(B\) is the Bulk Modulus (compressibility) and \(Q\) is the density. What happens to the speed if the density of the fluid increases?
• What must be true about the compressibility, \(B\), of water versus air, given that sound travels faster in water and water is denser than air?
• The speed of sound in a fluid is given by \(v=\sqrt{B/Q}\) where \(B\) is the Bulk Modulus (compressibility) and \(Q\) is the density. Can you think of a clever way to measure the Bulk Modulus of a fluid if you had an easy way to measure the speed of sound in a fluid? Explain.
• The speed of sound on a string is given by \(v=\sqrt{T/\mu}\) where \(T\) is the tension in Newtons and \(\mu\) is the linear density (thickness) in \(\text{kg/m}\). You also know that \(v=f\lambda\). Give two ways of changing the frequency of vibration of a guitar string based on the knowledge of these two equations.
• For the previous question, increasing the tension does what to the frequency? What does using a denser string do to the frequency?
• The following graph is of a wave, frozen in time at \(t = 0\). The equation describing the wave is \(y(x,t)=A\cos (kx-\omega t+\phi )\). Sketch the effect of doubling the amplitude, \(A\).

Figure \(\PageIndex{1}\)

• For the following graph of a wave, sketch the effect of doubling the wavelength.

Figure \(\PageIndex{2}\)

• The mathematical description of a sine wave is given by \(y(x,t)=A\cos (kx-\omega t+\phi )\). Explain what each of the terms \((A, k, \omega, \phi )\) represent.