Television broadcasts also use radio waves (see Figure 1.2). 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.
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 am- plitude, 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 1.2. 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 dont 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 1.2. 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.
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 1.1.
introducing radio waves
Electromagnetic waves on the left side of the Figure 1.1 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. Click image to the left or use the URL below. URL: Q: Based on the electromagnetic spectrum Figure 1.1, 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.
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electromagnetic waves vary in their wavelengths and frequencies.
--> a. true b. false
radio waves have the highest frequencies of all electromagnetic waves.
a. true --> b. false
radio waves have the least energy of all electromagnetic waves.
--> a. true b. false
radio waves vary in frequency from
--> a) 105 to 1012 hz. b) 1050 to 10,120 hz. c) 10,500 to 101,200 hz. d) none of the above
am radio waves
a) have longer wavelengths than fm radio waves. b) are encoded with signals by changing their amplitude. c) reflect off the ionosphere. --> d) all of the above.
in tv broadcasts
--> a) radio waves carry both sound and picture signals. b) sounds are encoded withy amplitude modulation. c) pictures are encoded with frequency modulation. d) all of the above
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