The Doppler Effect for sound results when relative motion of the source causes the wavelength of the sound waves to be decreased ahead of the source and stretched out behind (pitch is related to wavelength; the longer the wavelength, the lower the pitch).

Light is also a wave, and relative motion of the source leads to a corresponding Doppler effect for light. In this case it is not the pitch but the color (wavelength) that is shifted by the motion of the source. The wavelength is shifted to larger values if the motion of the source is away from the observer and to smaller values if the motion is toward the observer.