DOPPLER EFFECT

the shift in frequency of acoustic or electromagnetic radiation emitted by a source moving relative to an observer as perceived by the observer: the shift is to higher frequencies when the source approaches and to lower frequencies when it recedes. For example, when one hears the sound of the siren on a police car, the pitch is perceived to be going higher as the car approaches and going lower as the car recedes. Named after C.J. Doppler (1803–1853), Austrian physicist. —Handbook for Preclears Glossary (app 9.7.90)