Doppler effect [Christian J. Doppler; L, effectus] , the apparent change in frequency of sound or light waves emitted by a source as it moves away from or toward an observer. The frequency increases as the source moves toward the observer and decreases as it moves away, as the rising pitch of the whistle of an approaching train and the falling pitch of a departing train. The Doppler effect is also observed in electromagnetic radiation, such as light and radio waves. Also called Doppler shift. See also electromagnetic radiation, ultrasonography, wavelength.
