Physics

# 15.1The Electromagnetic Spectrum

Physics15.1 The Electromagnetic Spectrum

## Section Learning Objectives

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

• Define the electromagnetic spectrum, and describe it in terms of frequencies and wavelengths
• Describe and explain the differences and similarities of each section of the electromagnetic spectrum and the applications of radiation from those sections

## Teacher Support

### Teacher Support

• (7) Science concepts. The student knows the characteristics and behavior of waves. The student is expected to:
• (A) examine and describe oscillatory motion and wave propagation in various types of media;
• (B) investigate and analyze characteristics of waves, including velocity, frequency, amplitude, and wavelength, and calculate using the relationship between wave speed, frequency, and wavelength;
• (C) compare characteristics and behaviors of transverse waves, including electromagnetic waves and the electromagnetic spectrum, and characteristics and behaviors of longitudinal waves, including sound waves; and
• (F) describe the role of wave characteristics and behaviors in medical and industrial applications.

In addition, the High School Physics Laboratory Manual addresses content in this section in the lab titled: Light and Color, as well as the following standards:

• (7) Science concepts. The student knows the characteristics and behavior of waves. The student is expected to
• (C) compare characteristics and behaviors of transverse waves, including electromagnetic waves and the electromagnetic spectrum, and characteristics and behaviors of longitudinal waves, including sound waves.
• (8) Science concepts. The student knows simple examples of atomic, nuclear, and quantum phenomena. The student is expected to
• (B) compare and explain the emission spectra produced by various atoms.

## Section Key Terms

 electric field electromagnetic radiation (EMR) magnetic field Maxwell’s equations

## Teacher Support

### Teacher Support

[BL]Explain that the term spectrum refers to a physical property that has a broad range with values that are continuous in some cases and, in other cases, discrete. Ask for other examples of spectra, for example, sound, people’s heights, etc.

[OL]Ask students to name ways that sunlight affects Earth. Provide examples that students don’t name: photosynthesis, weather, climate, seasons, warming, etc. Discuss energy transformations that take place after light enters the atmosphere, such as transformations in food chains and ecosystems. Ask students if they can explain how the energy in fossil fuels was originally light energy.

The light we can see is called visible light. Dispel any misconceptions that visible light is somehow different from radiation we cannot see, except for frequency and wavelength. The fact that some radiation is visible has to do with how the eye functions, not with the radiation itself.

## The Electromagnetic Spectrum

We generally take light for granted, but it is a truly amazing and mysterious form of energy. Think about it: Light travels to Earth across millions of kilometers of empty space. When it reaches us, it interacts with matter in various ways to generate almost all the energy needed to support life, provide heat, and cause weather patterns. Light is a form of electromagnetic radiation (EMR). The term light usually refers to visible light, but this is not the only form of EMR. As we will see, visible light occupies a narrow band in a broad range of types of electromagnetic radiation.

## Teacher Support

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[OL]Discuss electric, magnetic, and gravitational fields. Point out how these three fields are similar, and how they differ.

[AL]Describe vectors as having magnitude and direction, and explain that fields are vector quantities. In these cases, the fields are made up of forces acting in a direction.

Electromagnetic radiation is generated by a moving electric charge, that is, by an electric current. As you will see when you study electricity, an electric current generates both an electric field, E, and a magnetic field, B. These fields are perpendicular to each other. When the moving charge oscillates, as in an alternating current, an EM wave is propagated. Figure 15.2 shows how an electromagnetic wave moves away from the source—indicated by the ~ symbol.

## Teacher Support

### Teacher Support

[BL]Review wave properties: frequency, wavelength, and amplitude. Ask students to recall sound and water waves, and explain how they relate to these properties.

[OL]Explain that an important difference between EM waves and other waves is that they can travel across empty space.

[AL]Ask if students remember the differences between longitudinal and transverse waves. Give examples. Explain that waves carry energy, not matter.

## Watch Physics

### Electromagnetic Waves and the Electromagnetic Spectrum

This video, link below, is closely related to the following figure. If you have questions about EM wave properties, the EM spectrum, how waves propagate, or definitions of any of the related terms, the answers can be found in this video.

## Grasp Check

In an electromagnetic wave, how are the magnetic field, the electric field, and the direction of propagation oriented to each other?

1. All three are parallel to each other and are along the x-axis.
2. All three are mutually perpendicular to each other.
3. The electric field and magnetic fields are parallel to each other and perpendicular to the direction of propagation.
4. The magnetic field and direction of propagation are parallel to each other along the y-axis and perpendicular to the electric field.

## Teacher Support

### Teacher Support

Direct students to use this video as a way of connecting to the information in the following two figures, as well as to the following table.

## Virtual Physics

### Radio Waves and Electromagnetic Fields

This simulation demonstrates wave propagation. The EM wave is propagated from the broadcast tower on the left, just as in Figure 15.2. You can make the wave yourself or allow the animation to send it. When the wave reaches the antenna on the right, it causes an oscillating current. This is how radio and television signals are transmitted and received.

## Grasp Check

Where do radio waves fall on the electromagnetic spectrum?

1. Radio waves have the same wavelengths as visible light.
2. Radio waves fall on the high-frequency side of visible light.
3. Radio waves fall on the short-wavelength side of visible light.
4. Radio waves fall on the low-frequency side of visible light.

## Teacher Support

### Teacher Support

Connect the discussion from the previous video, in which the generation of an electromagnetic wave is described, to this application of transmission and reception of electromagnetic waves. In particular, point out how the reception of the radio wave is essentially identical to the method by which the wave is generated. Explain also that these electromagnetic waves are the carrier waves on which audio or visual signals—either analog or digital—are placed, so that they can be transmitted to receivers within a certain range of the broadcast antenna.

Figure 15.2 A part of the electromagnetic wave sent out from an oscillating charge at one instant in time. The electric and magnetic fields (E and B) are in phase, and they are perpendicular to each other and to the direction of propagation. For clarity, the waves are shown only along one direction, but they propagate out in other directions too.

From your study of sound waves, recall these features that apply to all types of waves:

• Wavelength—The distance between two wave crests or two wave troughs, expressed in various metric measures of distance
• Frequency—The number of wave crests that pass a point per second, expressed in hertz (Hz or s–1)
• Amplitude: The height of the crest above the null point

As mentioned, electromagnetic radiation takes several forms. These forms are characterized by a range of frequencies. Because frequency is inversely proportional to wavelength, any form of EMR can also be represented by its range of wavelengths. Figure 15.3 shows the frequency and wavelength ranges of various types of EMR. With how many of these types are you familiar?

Figure 15.3 The electromagnetic spectrum, showing the major categories of electromagnetic waves. The range of frequencies and wavelengths is remarkable. The dividing line between some categories is distinct, whereas other categories overlap.

Take a few minutes to study the positions of the various types of radiation on the EM spectrum, above. The narrow band that is visible light extends from lower-frequency red light to higher-frequency violet light. Frequencies just below the visible are called infrared (below red) and those just above are ultraviolet (beyond violet). Radio waves, which overlap with the frequencies used for media broadcasts of TV and radio signals, occupy frequencies even lower than infrared (IR). The microwave radiation that you see on the diagram is the same radiation that is used in a microwave oven. What we feel as radiant heat is also a form of low-frequency EMR. The high-frequency radiation to the right of ultraviolet (UV) includes X-rays and gamma (γ) rays.

## Teacher Support

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[BL]Notice that most harmful forms of EM radiation are on the high-frequency end of the spectrum.

Heat waves, a type of infrared radiation, are basically no different from other EM waves. We feel them as heat because they have a frequency that interacts with our bodies in a way that transforms EM energy into thermal energy.

## Boundless Physics

### Maxwell’s Equations

The Scottish physicist James Clerk Maxwell (1831–1879) is regarded widely to have been the greatest theoretical physicist of the nineteenth century. Although he died young, Maxwell not only formulated a complete electromagnetic theory, represented by Maxwell’s equations, he also developed the kinetic theory of gases, and made significant contributions to the understanding of color vision and the nature of Saturn’s rings.

Maxwell brought together all the work that had been done by brilliant physicists, such as Ørsted, Coulomb, Ampere, Gauss, and Faraday, and added his own insights to develop the overarching theory of electromagnetism. Maxwell’s equations are paraphrased here in words because their mathematical content is beyond the level of this text. However, the equations illustrate how apparently simple mathematical statements can elegantly unite and express a multitude of concepts—why mathematics is the language of science.

Maxwell’s Equations

1. Electric field lines originate on positive charges and terminate on negative charges. The electric field is defined as the force per unit charge on a test charge, and the strength of the force is related to the electric constant, ε0.
2. Magnetic field lines are continuous, having no beginning or end. No magnetic monopoles are known to exist. The strength of the magnetic force is related to the magnetic constant, μ0.
3. A changing magnetic field induces an electromotive force (emf) and, hence, an electric field. The direction of the emf opposes the change, changing direction of the magnetic field.
4. Magnetic fields are generated by moving charges or by changing electric fields.

Maxwell’s complete theory shows that electric and magnetic forces are not separate, but different manifestations of the same thing—the electromagnetic force. This classical unification of forces is one motivation for current attempts to unify the four basic forces in nature—the gravitational, electromagnetic, strong nuclear, and weak nuclear forces. The weak nuclear and electromagnetic forces have been unified, and further unification with the strong nuclear force is expected; but, the unification of the gravitational force with the other three has proven to be a real head-scratcher.

One final accomplishment of Maxwell was his development in 1855 of a process that could produce color photographic images. In 1861, he and photographer Thomas Sutton worked together on this process. The color image was achieved by projecting red, blue, and green light through black-and-white photographs of a tartan ribbon, each photo itself exposed in different-colored light. The final image was projected onto a screen (see Figure 15.4).

Figure 15.4 Maxwell and Sutton’s photograph of a colored ribbon. This was the first durable color photograph. The plaid tartan of the Scots made a colorful photographic subject.

## Teacher Support

### Teacher Support

Features that encouraged mathematicians and physicists to accept Maxwell’s equations is that they are seen as being both elegant and—considering the difference between an electric charge and a magnetic dipole, which give rise to the respective fields—essentially symmetrical. When scientists are looking for an approach to developing a new theory, they usually begin with the simplest and most symmetrical explanations. An example of such symmetry is the fact that electrons and protons have equal and opposite charges. You can see the symmetry in the four statements, given above, that describe the equations. The first two statements show a similar treatment of electric and magnetic fields, and the last two describe how a magnetic field can generate an electric field, and vice versa.

From our present-day perspective, we can now see the significance of Maxwell’s equations. This was the first step in the quest to unify all natural forces under one theory. After Maxwell unified the electric and magnetic forces as the electromagnetic force, others unified this force with the weak nuclear force, and there is evidence that the strong nuclear force can be unified with the electroweak force. The only force that has resisted unification with the others is the gravitational force. A theory that would unify all forces is often referred as a grand unified theory or a theory of everything. The quest for such a theory is still underway.

Describe electromagnetic force as explained by Maxwell’s equations.
1. According to Maxwell’s equations, electromagnetic force gives rise to electric force and magnetic force.
2. According to Maxwell’s equations, electric force and magnetic force are different manifestations of electromagnetic force.
3. According to Maxwell’s equations, electric force is the cause of electromagnetic force.
4. According to Maxwell’s equations, magnetic force is the cause of electromagnetic force.

All the EM waves mentioned above are basically the same form of radiation. They can all travel across empty space, and they all travel at the speed of light in a vacuum. The basic difference between types of radiation is their differing frequencies. Each frequency has an associated wavelength. As frequency increases across the spectrum, wavelength decreases. Energy also increases with frequency. Because of this, higher frequencies penetrate matter more readily. Some of the properties and uses of the various EM spectrum bands are listed in Table 15.1.

## Teacher Support

### Teacher Support

[BL]Explain transparency and opacity. Discuss how some materials are transparent to certain frequencies but opaque to others. Ask students for examples of materials that can be penetrated by some EM frequencies but not by others. Ask for examples of materials that are transparent to visible light and materials that are opaque to visible light.

[OL]Ask students why a lead apron is laid across dental patients during dental X-rays. Explain that X-rays are at the high-energy end of the spectrum and that they are very penetrating. They are only stopped by very dense materials, such as lead.

[AL]Ask if students can explain Earth’s greenhouse effect in terms of the penetrating power of various frequencies of EM radiation. Explain that the atmosphere is more transparent to visible light than to heat waves. Visible light penetrates the atmosphere and warms Earth’s surface. The heated surface radiates heat waves, which are trapped partially by certain gases in the atmosphere.

Types of EM Waves Production Applications Life Sciences Aspect Issues
Radio and TV Accelerating charges Communications, remote controls MRI Requires controls for band use
Microwaves Accelerating charges & thermal agitation Communications, microwave ovens, radar Deep heating Cell phone use
Infrared Thermal agitation & electronic transitions Thermal imaging, heating Absorption by atmosphere Greenhouse effect
Visible Light Thermal agitation & electronic transitions All pervasive Photosynthesis, human vision
Ultraviolet Thermal agitation & electronic transitions Sterilization, slowing abnormal growth of cells Vitamin D production Ozone depletion, causes cell damage
X-rays Inner electronic transitions & fast collisions Medical, security Medical diagnosis, cancer therapy Causes cell damage
Gamma Rays Nuclear decay Nuclear medicine, security Medical diagnosis, cancer therapy Causes cell damage, radiation damage
Table 15.1 Electromagnetic Waves This table shows how each type of EM radiation is produced, how it is applied, as well as environmental and health issues associated with it.

The narrow band of visible light is a combination of the colors of the rainbow. Figure 15.5 shows the section of the EM spectrum that includes visible light. The frequencies corresponding to these wavelengths are at the red end to at the violet end. This is a very narrow range, considering that the EM spectrum spans about 20 orders of magnitude.

Figure 15.5 A small part of the electromagnetic spectrum that includes its visible components. The divisions between infrared, visible, and ultraviolet are not perfectly distinct, nor are the divisions between the seven rainbow colors

## Teacher Support

### Teacher Support

[BL]Review the primary and secondary colors of pigments. Note that this is subtractive color mixing.

[OL]Explain the difference between subtractive and additive color mixing. The colors on the subtractive color wheel are made by pigments that absorb all colors but one. Therefore, when these colors all overlap, all light is absorbed and the result is black. White light is a combination of all colors, so when all colors are added together on the additive color wheel, the result is white. Explain that cyan is a shade of blue and that magenta is a shade of red.

## Tips For Success

Wavelengths of visible light are often given in nanometers, nm. One nm equals $10 −9 10 −9$ m. For example, yellow light has a wavelength of about 600 nm, or $6× 10 −7 6× 10 −7$ m.

As a child, you probably learned the color wheel, shown on the left in Figure 15.6. It helps if you know what color results when you mix different colors of paint together. Mixing two of the primary pigment colors—magenta, yellow, or cyan—together results in a secondary color. For example, mixing cyan and yellow makes green. This is called subtractive color mixing. Mixing different colors of light together is quite different. The diagram on the right shows additive color mixing. In this case, the primary colors are red, green, and blue, and the secondary colors are cyan, magenta, and yellow. Mixing pigments and mixing light are different because materials absorb light by a different set of rules than does the perception of light by the eye. Notice that, when all colors are subtracted, the result is no color, or black. When all colors are added, the result is white light. We see the reverse of this when white sunlight is separated into the visible spectrum by a prism or by raindrops when a rainbow appears in the sky.

Figure 15.6 Mixing colored pigments follows the subtractive color wheel, and mixing colored light follows the additive color wheel.

## Virtual Physics

### Color Vision

This video demonstrates additive color and color filters. Try all the settings except Photons.

PhET Explorations: Color Vision. Make a whole rainbow by mixing red, green, and blue light. Change the wavelength of a monochromatic beam or filter white light. View the light as a solid beam, or see the individual photons.
Ordinary white light is a combination of all colors of visible light. How would a blue absorption filter placed in front of a white light source affect the light you observe?
1. A blue filter absorbs blue light, causing the observed light to be a combination of the other colors.
2. A blue filter absorbs the opposite color of light—orange, causing the observed light to be blue.
3. A blue filter permits only blue light to pass though, absorbing the other colors and leaving blue light for the observer.
4. A blue filter permits only the opposite color light—orange—to pass through, leaving orange light for the observer.

## Teacher Support

### Teacher Support

Have students adjust the different colored lights for the RGB bulb simulation, first with individual settings, then with combinations of two and three colors to see what colors result and are perceived. Similarly, with the Single Bulb simulation, have students note how different filter settings affect what colors are seen for light with different color components.

Humans have found uses for every part of the electromagnetic spectrum. We will take a look at the uses of each range of frequencies, beginning with visible light. Most of our uses of visible light are obvious; without it our interaction with our surroundings would be much different. We might forget that nearly all of our food depends on the photosynthesis process in plants, and that the energy for this process comes from the visible part of the spectrum. Without photosynthesis, we would also have almost no oxygen in the atmosphere.

## Teacher Support

### Teacher Support

[BL]Ask how different frequencies of EM radiation are applied. Name each frequency range, and ask the students to supply the application, for example, X-rays used in medical imaging.

[OL]Ask students if they know why low-frequency radiation generally has different uses than high-frequency radiation. Explain that it has to do with penetrating power, which is related to health hazards.

[AL]Mention the ranges of TV signals designated very high frequency (VHF) and ultrahigh frequency (UHF). Explain that these frequencies are only relatively high compared to radio broadcast frequencies. Their place in the whole EM spectrum is at the low end.

The low-frequency, infrared region of the spectrum has many applications in media broadcasting. Television, radio, cell phone, and remote-control devices all broadcast and/or receive signals with these wavelengths. AM and FM radio signals are both low-frequency radiation. They are in different regions of the spectrum, but that is not their basic difference. AM and FM are abbreviations for amplitude modulation and frequency modulation. Information in AM signals has the form of changes in amplitude of the radio waves; information in FM signals has the form of changes in wave frequency.

Another application of long-wavelength radiation is found in microwave ovens. These appliances cook or warm food by irradiating it with EM radiation in the microwave frequency range. Most kitchen microwaves use a frequency of $2.45× 10 9 2.45× 10 9$ Hz. These waves have the right amount of energy to cause polar molecules, such as water, to rotate faster. Polar molecules are those that have a partial charge separation. The rotational energy of these molecules is given up to surrounding matter as heat. The first microwave ovens were called Radaranges because they were based on radar technology developed during World War II.

Radar uses radiation with wavelengths similar to those of microwaves to detect the location and speed of distant objects, such as airplanes, weather formations, and motor vehicles. Radar information is obtained by receiving and analyzing the echoes of microwaves reflected by an object. The speed of the object can be measured using the Doppler shift of the returning waves. This is the same effect you learned about when you studied sound waves. Like sound waves, EM waves are shifted to higher frequencies by an object moving toward an observer, and to lower frequencies by an object moving away from the observer. Astronomers use this same Doppler effect to measure the speed at which distant galaxies are moving away from us. In this case, the shift in frequency is called the red shift, because visible frequencies are shifted toward the lower-frequency, red end of the spectrum.

Exposure to any radiation with frequencies greater than those of visible light carries some health hazards. All types of radiation in this range are known to cause cell damage. The danger is related to the high energy and penetrating ability of these EM waves. The likelihood of being harmed by any of this radiation depends largely on the amount of exposure. Most people try to reduce exposure to UV radiation from sunlight by using sunscreen and protective clothing. Physicians still use X-rays to diagnose medical problems, but the intensity of the radiation used is extremely low. Figure 15.8 shows an X-ray image of a patient’s chest cavity.

One medical-imaging technique that involves no danger of exposure is magnetic resonance imaging (MRI). MRI is an important imaging and research tool in medicine, producing highly detailed two- and three-dimensional images. Radio waves are broadcast, absorbed, and reemitted in a resonance process that is sensitive to the density of nuclei, usually hydrogen nuclei—protons.

Figure 15.8 This shadow X-ray image shows many interesting features, such as artificial heart valves, a pacemaker, and wires used to close the sternum. (credit: P.P. Urone)

## Teacher Support

### Teacher Support

Use these questions to assess student achievement of the section’s Learning Objectives. If students are struggling with a specific objective, these questions will help identify any gaps and direct students to the relevant content.

1.

Identify the fields produced by a moving charged particle.

1. Both an electric field and a magnetic field will be produced.
2. Neither a magnetic field nor an electric field will be produced.
3. A magnetic field, but no electric field will be produced.
4. Only the electric field, but no magnetic field will be produced.
2 .
X-rays carry more energy than visible light. Compare the frequencies and wavelengths of these two types of EM radiation.
1. Visible light has higher frequencies and shorter wavelengths than X-rays.
2. Visible light has lower frequencies and shorter wavelengths than X-rays.
3. Visible light has higher frequencies and longer wavelengths than X-rays.
4. Visible light has lower frequencies and longer wavelengths than X-rays.
3 .
How does wavelength change as frequency increases across the EM spectrum?
1. The wavelength increases.
2. The wavelength first increases and then decreases.
3. The wavelength first decreases and then increases.
4. The wavelength decreases.
4 .
Why are X-rays used in imaging of broken bones, rather than radio waves?
1. X-rays have higher penetrating energy than radio waves.
2. X-rays have lower penetrating energy than radio waves.
3. X-rays have a lower frequency range than radio waves.
4. X-rays have longer wavelengths than radio waves.
5 .
Identify the fields that make up an electromagnetic wave.
1. both an electric field and a magnetic field
2. neither a magnetic field nor an electric field
3. only a magnetic field, but no electric field
4. only an electric field, but no magnetic field