Doppler Effect

As sound or light propagates, the Doppler effect is the perceived change in the frequency of a wavelength by an observer moving relative to the source of the wave. The Doppler effect is shown as a shift in the visible light spectrum. Due to the expansion of the universe, distant galaxies move away from us at a speed proportional to their distance from us, which is why the Doppler red shift can be used to measure those distances. This Demonstration also shows how a sound wave produces a higher or lower frequency as an object approaches or moves away from the source.