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Chapter 1. Energy
1.44 Electromagnetic Spectrum
• Describe electromagnetic radiation and its properties.
• Give an overview of the electromagnetic spectrum.
It’s a warm sunny Saturday, and Michael and Lavar have a big day planned. They’re going to ride across town to
meet their friends and then go to the zoo. The boys may not realize it, but they will be bombarded by electromagnetic
radiation as they ride their bikes and walk around the zoo grounds. The only kinds of radiation they can detect are
visible light, which allows them to see, and infrared light, which they feel as warmth on their skin.
Q: Besides visible light and infrared light, what other kinds of electromagnetic radiation will the boys be exposed to
in sunlight?
A: Sunlight consists of all the different kinds of electromagnetic radiation, from harmless radio waves to deadly
gamma rays. Fortunately, Earth’s atmosphere prevents most of the harmful radiation from reaching Earth’s surface.
You can read about the different kinds of electromagnetic radiation in this article.
Electromagnetic Radiation
Electromagnetic radiation is energy that travels in waves across space as well as through matter. Most of the
electromagnetic radiation on Earth comes from the sun. Like other waves, electromagnetic waves are characterized
by certain wavelengths and wave frequencies. Wavelength is the distance between two corresponding points on
adjacent waves. Wave frequency is the number of waves that pass a fixed point in a given amount of time.
Electromagnetic waves with shorter wavelengths have higher frequencies and more energy.
A Spectrum of Electromagnetic Waves
Visible light and infrared light are just a small part of the full range of electromagnetic radiation, which is called the
electromagnetic spectrum. You can see the waves of the electromagnetic spectrum in the Figure 1.91. At the top
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1.44. Electromagnetic Spectrum
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of the diagram, the wavelengths of the waves are given. Also included are objects that are about the same size as the
corresponding wavelengths. The frequencies and energy levels of the waves are shown at the bottom of the diagram.
Some sources of the waves are also given. For a video introduction to the electromagnetic spectrum, go to this URL:
http://www.youtube.com/watch?NR=1&feature=endscreen&v=cfXzwh3KadE
FIGURE 1.91
• On the left side of the electromagnetic spectrum diagram are radio waves and microwaves. Radio waves have
the longest wavelengths and lowest frequencies of all electromagnetic waves. They also have the least amount
of energy.
• On the right side of the diagram are X rays and gamma rays. They have the shortest wavelengths and highest
frequencies of all electromagnetic waves. They also have the most energy.
• Between these two extremes are waves that are commonly called light. Light includes infrared light, visible
light, and ultraviolet light. The wavelengths, frequencies, and energy levels of light fall in between those of
radio waves on the left and X rays and gamma rays on the right.
Q: Which type of light has the longest wavelengths?
A: Infrared light has the longest wavelengths.
Q: What sources of infrared light are shown in the diagram?
A: The sources in the diagram are people and light bulbs, but all living things and most other objects give off infrared
light.
Summary
• Electromagnetic radiation travels in waves through space or matter. Electromagnetic waves with shorter
wavelengths have higher frequencies and more energy.
• The full range of electromagnetic radiation is called the electromagnetic spectrum. From longest to shortest
wavelengths, it includes radio waves, microwaves, infrared light, visible light, ultraviolet light, X rays, and
gamma rays.
Vocabulary
• electromagnetic spectrum: Full range of wavelengths of electromagnetic waves, from radio waves to gamma
rays.
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Chapter 1. Energy
Practice
At the first URL below, read about electromagnetic waves with different frequencies. Then use the information to
complete the table at the second URL. http://www.darvill.clara.net/emag/index.htm and http://www.darvill.clara.net
/emag/gcseemag.pdf
Review
1.
2.
3.
4.
5.
6.
Describe the relationship between the wavelength and frequency of electromagnetic waves.
What is the electromagnetic spectrum?
Which electromagnetic waves have the longest wavelengths?
Identify a source of microwaves.
Which type of light has the highest frequencies?
Explain why gamma rays are the most dangerous of all electromagnetic waves.
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1.45. Radio Waves
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1.45 Radio Waves
• Give an overview of the electromagnetic spectrum.
• Describe radio waves and identify their uses.
• Explain how radio waves are used for radio and television broadcasts.
Raymond can’t help singing along when his favorite song starts playing on the radio. Do you like to listen to the
radio? Did you ever wonder how music travels from a radio station to a radio receiver? How do the sounds travel
through air and buildings and everything else in between you and the station? The answer is by electromagnetic
waves.
A Spectrum of Waves
Electromagnetic waves consist of vibrating electric and magnetic fields. They transfer energy across space as well
as through matter. Electromagnetic waves vary in their wavelengths and frequencies, and higher-frequency waves
have more energy. The full range of wavelengths of electromagnetic waves is called the electromagnetic spectrum.
It is outlined in the following Figure 1.92.
Introducing Radio Waves
Electromagnetic waves on the left side of the diagram above are called radio waves. Radio waves are electromagnetic waves with the longest wavelengths. They may have wavelengths longer than a soccer field. They are also
the electromagnetic waves with the lowest frequencies. With their low frequencies, they have the least energy of all
electromagnetic waves. Nonetheless, radio waves are very useful. They are used for radio and television broadcasts
and many other purposes. You can learn more about radio waves, including how they were discovered, at this URL:
http://www.youtube.com/watch?v=al7sFP4C2TY
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Chapter 1. Energy
FIGURE 1.92
Q: Based on the electromagnetic spectrum diagram above, what is the range of frequencies of radio waves?
A: The range of frequencies of radio waves is between 105 and 1012 Hz, or waves per second.
AM and FM Radio
In radio broadcasts, sounds are encoded in radio waves, and then the waves are sent out through the atmosphere from
a radio tower. A radio receiver detects the waves and changes them back to sounds. You may have listened to both
AM and FM radio stations. How sounds are encoded in radio waves differs between AM and FM broadcasts.
• AM stands for amplitude modulation. In AM broadcasts, sound signals are encoded by changing the amplitude, or maximum height, of radio waves. AM broadcasts use longer wavelength radio waves than FM
broadcasts. Because of their longer wavelengths, AM waves reflect off a layer of the upper atmosphere called
the ionosphere. You can see how this happens in the Figure 1.93. Because the waves are reflected, they can
reach radio receivers that are very far away from the radio tower.
• FM stands for frequency modulation. In FM broadcasts, sound signals are encoded by changing the frequency
of radio waves. Frequency modulation allows FM waves to encode more information than does amplitude
modulation, so FM broadcasts usually produce clearer sounds than AM broadcasts. However, the relatively
short wavelengths of FM waves means that they don’t reflect off the ionosphere as AM waves do. Instead,
FM waves pass through the ionosphere and out into space. This is also shown in the Figure 1.93 below. As a
result, FM waves cannot reach very distant receivers.
Q: The composition of the ionosphere changes somewhat from day to night. The changes make the nighttime
ionosphere even better at reflecting AM radio waves. How do you think this might affect the distance AM radio
waves travel at night?
A: With greater reflection off the ionosphere, AM waves can travel even farther at night than they can during the
day. Radio receivers can often pick up radio broadcasts at night from cities that are hundreds of miles away.
Television
Television broadcasts also use radio waves (see diagram above). For TV broadcasts, sounds are encoded with
frequency modulation, and pictures are encoded with amplitude modulation. The encoded waves are broadcast from
a TV tower. When the waves are received by television sets, they are decoded and changed back to sounds and
pictures.
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FIGURE 1.93
Summary
• Electromagnetic waves vary in their wavelengths, frequencies, and energy levels. The full range of electromagnetic waves makes up the electromagnetic spectrum.
• Radio waves are electromagnetic waves with the longest wavelengths, lowest frequencies, and least amount
of energy. They are used for radio and television broadcasts and many other purposes.
• In radio broadcasts, sounds are encoded in radio waves by changing either the amplitude (AM) or frequency
(FM) of the waves. The encoded waves are broadcast from a tower and changed back to sounds by radio
receivers.
• In television broadcasts, sounds and pictures are encoded in radio waves, broadcast from a tower, and changed
back to sounds and pictures by television sets.
Vocabulary
• radio wave: Any wave in the electromagnetic spectrum that has a wavelength longer than infrared light.
Practice
Observe the three radio waves in the animation at the following URL. Then write a short paragraph comparing and
contrasting the three waves. http://en.wikipedia.org/wiki/File:Amfm3-en-de.gif
Review
1. What are radio waves?
2. Compare and contrast AM and FM radio broadcasts.
3. Explain how radio waves are used to encode sounds and pictures in television broadcasts.
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