SAT Physics Subject Test

Chapter 13 Waves

THE DOPPLER EFFECT FOR LIGHT

Light, or electromagnetic waves, also experience the Doppler effect. As with sound, motion toward corresponds to a frequency shift upward and motion away corresponds to a frequency shift downward. Because of special relativity (discussed in Chapter 16), the wavelength will change regardless of who moves.

21. Light from a distant galaxy is received on Earth with a wavelength of 650 nm. It’s known that the wavelength of this light upon emission was 625 nm. Is the galaxy moving toward us or away from us?

Here’s How to Crack It

The fact that the light has a longer wavelength upon detection than it had at emission tells us that the source is receding. Astronomers would say that the light has been red-shifted—that is, the wavelength was shifted upward, toward the red end of the visible spectrum.