Questions about the change in frequency and wavelength of an electromagnetic wave caused by a non-zero relative velocity of a source relative to the observer.

The Doppler effect is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.

A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. The Doppler effect in astronomy results in either a so-called redshift or blueshift. It is used to measure the speed at which stars and galaxies are approaching or receding Earth.