Doppler shift

The phenomenon in which the wavelength of a wave is compressed ("blue shifted") to an observer approaching the wave's source, and extended ("red shifted") to one retreating from the wave's source (the perceived frequency thus being raised or lowered respectively). In radiocommuncations it becomes significant with high-velocity sources such as satellites and other spacecraft. A relative speed difference of 100 km/h (about 62 mph) will cause a perceived difference of about 28 Hz in a 300-MHz signal.

The corresponding phenomenon with sound waves is easily observed audibly with, for example, vehicle horns.