User:John R. Brews/Sample: Difference between revisions
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==Doppler effect== | ==Doppler effect== | ||
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{{Image|Doppler effect.PNG|right|250px|Boat traveling against waves experiences Doppler effect.}} | |||
The '''Doppler effect''' (or '''Doppler shift''', or '''Doppler's principle'''), named after [[Christian Doppler]] who proposed it in 1842,<ref name="AlecEden"> | The '''Doppler effect''' (or '''Doppler shift''', or '''Doppler's principle'''), named after [[Christian Doppler]] who proposed it in 1842,<ref name="AlecEden"> | ||
{{cite journal |author=C Doppler|title=Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels (On the colored light of the [[binary stars]] and some other stars of the heavens)|journal=Abhandlungen der koniglich bohmischen Gesellschaft der Wissenschaften|volume=vol 2, |pages=pp. 465-482 |year=1843}} | {{cite journal |author=C Doppler|title=Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels (On the colored light of the [[binary stars]] and some other stars of the heavens)|journal=Abhandlungen der koniglich bohmischen Gesellschaft der Wissenschaften|volume=vol 2, |pages=pp. 465-482 |year=1843}} | ||
Revision as of 13:31, 12 April 2011
Doppler effect
The Doppler effect (or Doppler shift, or Doppler's principle), named after Christian Doppler who proposed it in 1842,[1] is the change 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. The received 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.
The relative increase in frequency can be explained as follows. When the source of the waves is moving toward the observer, each successive wave crest is emitted from a position closer to the observer than the previous wave. Therefore each wave takes slightly less time to reach the observer than the previous wave. Therefore the time between the arrival of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wavefronts is reduced; so the waves "bunch together". Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wavefronts is increased, so the waves "spread out".
For waves that propagate in a medium, 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 of the 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.
Notes
- ↑ C Doppler (1843). "Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels (On the colored light of the binary stars and some other stars of the heavens)". Abhandlungen der koniglich bohmischen Gesellschaft der Wissenschaften vol 2,: pp. 465-482.