The Doppler effect is the apparent difference between the frequency at which sound or light waves leave a source and that at which they reach an observer, caused by relative motion of the observer and the wave source.

The Doppler effect for sound was first described by the Austrian physicist Christian Doppler in 1842. He presented his ideas in a paper titled “On the Coloured Light of the Double Stars and Certain Other Stars of the Heavens,” proposing that the observed frequency of waves depends on the relative speed of the source and the observer. This principle was later confirmed experimentally by the Dutch scientist Christophorus Buys Ballot in 1845.