Doppler effect |
Change of wavelength caused by motion of the source
The Doppler effect (or Doppler shift), named after Austrian physicist Christian Doppler who proposed it in 1842, is thechange in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from an observer. Thereceived frequency is higher (compared to the emitted frequency) during the approach, it is identical at the instant of passing by, and it is lower during the recession.
For waves that propagate in amedium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion ofthe source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as light or gravity in general relativity,only the relative difference in velocity between the observer and the source needs to be considered.
Doppler first proposed the effect in 1842 in his treatise "Über das farbige Licht der Doppelsterneund einiger anderer Gestirne des Himmels" (On the coloured light of the binary stars and some other stars of the heavens). The hypothesis was tested for sound waves by Buys Ballot in 1845. Heconfirmed that the sound's pitch was higher than the emitted frequency when the sound source approached him, and lower than the emitted frequency when the sound source receded from him. Hippolyte Fizeaudiscovered independently the same phenomenon on electromagnetic waves in 1848 (in France, the effect is sometimes called "effet Doppler-Fizeau"). In Britain, John Scott Russell made an experimental studyof the Doppler effect (1848).
An English translation of Doppler's 1842 treatise can be found in the book The Search for Christian Doppler by Alec Eden
The boundary layer...
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