The Doppler effect is a change in the
frequency of a wave (sound, light, or other waves) that takes
place whenever relative motion exists between the source and a
detector. This effect takes its name from the Austrian physicist
Christian Johann Doppler, who first stated the physical principal
A familiar example of this phenomenon, is the change in pitch that can be heard by an observer listening to the whistle of an approaching (or receding) train. In fact, in this case, Doppler's principle explains why, if the source of sound (emitting a constant pitch) is moving toward an observer, the sound seems higher in pitch, whereas if the source is moving away it seems lower.