Doppler shift

Doppler shift, or the Doppler effect, is the change in frequency of a wave – be it electromagnetic, sound or else –  due to relative motion between the source and the emitter. The effect is named after the Austrian physicist Christian Doppler, who described the phenomena in 1842.

The Doppler effect is commonly seen in everyday life when the pitch of a  passing vehicle changes as it passes you. The vehicle can be heard to have a higher pitch when approaching and then a lower pitch when the vehicle is going away from you.