Doppler Effect and Electromagnetic Waves for AP Physics B & C

Practice problems for these concepts can be found at:

Waves Practice Problems for AP Physics B & C

Doppler Effect

Whenever a fire engine or ambulance races by you with its sirens blaring, you experience the Doppler effect. Similarly, if you enjoy watching auto racing, that "Neeee-yeeeer" you hear as the cars scream past the TV camera is also attributable to the Doppler effect.

To understand the Doppler effect, let's look at what happens as a fire truck travels past you (Figures 23.10a and 23.10b).

As the fire truck moves toward you, its sirens are emitting sound waves. Let's say that the sirens emit one wave pulse when the truck is 50 meters away from you. It then emits another pulse when the truck is 49.99 meters away from you. And so on. Because the truck keeps moving toward you as it emits sound waves, it appears to you that these waves are getting scrunched together.

Then, once the truck passes you and begins to move away from you, it appears as if the waves are stretched out.

Now, imagine that you could record the instant that each sound wave hit you. When the truck was moving toward you, you would observe that the time between when one wave hit and when the next wave hit was very small. However, when the truck was moving away from you, you would have to wait a while between when one wave hit you and when the next one did. In other words, when the truck was moving toward you, you registered that the sirens were making a higher frequency noise; and when the truck was moving away, you registered that the sirens were making a lower frequency noise.

That's all you really need to know about the Doppler effect. Just remember, the effect is rather small—at normal speeds, the frequency of, say, a 200 Hz note will only change by a few tens of Hz, not hundreds of Hz.

Electromagnetic Waves

When radio waves are beamed through space, or when X-rays are used to look at your bones, or when visible light travels from a light bulb to your eye, electromagnetic waves are at work. All these types of radiation fall in the electromagnetic spectrum, shown in Figure 23.11.

The unique characteristic about electromagnetic waves is that all of them travel at exactly the same speed through a vacuum—3 × 108 m/s. The more famous name for "3 × 10⁸ m/s" is "the speed of light," or "c."

What makes one form of electromagnetic radiation different from another form is simply the frequency of the wave. AM radio waves have a very low frequency and a very long wavelength; whereas, gamma rays have an extremely high frequency and an exceptionally short wavelength. But they're all just varying forms of light waves.

Practice problems for these concepts can be found at:

Waves Practice Problems for AP Physics B & C