Doppler effect
The shift in observed frequency when a wave source and observer move relative to each other.
Definition
When a source of waves moves relative to its medium, the wavefronts emitted ahead of it bunch up and those behind it stretch apart. An observer in front of the source intercepts wavefronts more often than they are emitted — a higher frequency. An observer behind the source intercepts them less often — a lower frequency.
The effect applies to every kind of wave: sound, light, water, seismic. For sound, the formula is asymmetric in the source and observer velocities because the medium fixes a preferred frame. For light in vacuum, no preferred frame exists, and the shift depends only on the relative velocity.
History
Predicted by Christian Doppler in 1842 and experimentally confirmed by Christophe Buys Ballot in 1845 using trumpet players on a moving railway flatcar.