DOPPLER EFFECT, the change in the observed frequency of a vibration owing to relative motion between the observer and the source of the vibration. In sound (q.v.) this effect is an everyday experience ; e.g., on passing a ringing bell at any speed above about io m.p.h. the pitch of the note suddenly sounds lower. The Doppler effect for light waves evident in spectroscopy (q.v.; see also LIGHT: Waves and Interference).