Apparent difference between the frequency at which waves
including light, sound, and radio waves
leave a source and that at which they reach an observer.
The effect, first described by the Austrian physicist Christian Doppler (18031853), is caused by the relative motion of the observer and the wave source. It can be observed by listening to the blowing horn or siren of an approaching vehicle, whose pitch rises as the vehicle approaches the observer and falls as it recedes. It is used in radar and to calculate the speed of stars by observing the change in frequency of their light.