Apparent change in frequency of a wave caused by motion of a source with respect to a receiver and/or vice-versa. Frequencies are increased if the source and receiver are approaching, decreased if they are moving apart; see Figure D-21. For sound and water waves, the observed frequency is given in terms of the source frequency the velocity of propagation , the velocity of the source , and the velocity of the observer (components of velocity toward each other):
The relationship for light and radio waves is slightly different because of relativistic effects and the constancy of the velocity of electromagnetic waves irrespective of any motion of source and observer:
See Doppler navigation. Named for Christian Johann Doppler (1803–1853), Austrian physicist.