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Illustration 32.2: Wave Crests

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In the 19th century it was discovered that a moving charge produces an electromagnetic wave. Rapidly oscillating charged particles (such as electrons in an atom) produce visible light, while slowly oscillating charges (such as those in an antenna) produce radio waves. Although waves with different frequencies produce different effects when they interact with matter, their propagation through space is quite similar. These similarities are the subject of this Illustration. Restart.

Electromagnetic waves have regions of high- and low-field strengths that are analogous to the high and low pressure regions of a sound wave. Analogies between electromagnetic waves and sound waves can be useful, but they should not be pushed too far. This Illustration shows one such analogy. An oscillating charge within the back circle produces a wave, and this wave is seen to propagate away from the source. The wave crests and troughs moving away from the source represent regions of strong electric field. The troughs are regions where the electric field is also strong, but the field is pointing in the opposite direction from the field at the crests. As waves propagate away from the source, their amplitude decreases. The red wave traveling to the right illustrates this.

Electromagnetic waves are different from sound waves, and this Illustration does little to point out this difference. Sound requires a medium for propagation, whereas electromagnetic waves do not need a medium: They can propagate in a vacuum. Furthermore, electric fields cannot propagate energy without a complementary magnetic field. The magnetic field associated with an electromagnetic wave is perpendicular to the electric field and is not shown. But the wavelength, frequency, and amplitude of the electric field are correctly illustrated and provide clues to the following questions:

• Do the frequency and period of the electromagnetic wave depend on the distance from the source?
• How does electric field amplitude depend on the distance from the source?
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