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.
- 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.