SAT Physics Subject Test

Chapter 13 Waves


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?

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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.