Doppler effect

The Doppler effect, named after Christian Andreas Doppler, is the apparent change in frequency or wavelength of a wave that is perceived by an observer moving relative to the source of the waves. For waves, such as sound waves, that propagate in a wave medium, the velocity of the observer and the source are reckoned relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from either motion of the source or motion of the observer. Each of these effects is analyzed separately.

Related Topics:
Christian Andreas Doppler - Frequency - Wavelength - Wave - Sound

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Doppler first proposed the effect in 1842 in the monograph Über das farbige Licht der Doppelsterne und einige andere Gestirne des Himmels (On the colored light of the binary star and other stars). The hypothesis was tested for sound waves by the Dutch scientist Christoph Hendrik Diederik Buys Ballot in 1845. He confirmed that the sound's pitch was higher as the sound source approached him, and lower as the sound source receded from him. Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848 (in France, the effect is sometimes called "effet Doppler-Fizeau").

Related Topics:
1842 - Dutch - Christoph Hendrik Diederik Buys Ballot - 1845 - Hippolyte Fizeau - Electromagnetic wave - 1848 - France

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It is important to realize that the frequency of the sounds that the source emits does not actually change. To understand what happens, consider the following analogy. Someone throws one ball every second in your direction. Assume that balls travel with constant velocity. If the thrower is stationary, you will receive one ball every second. However, if he is moving towards you, you will receive balls more frequently than that because there will be less spacing between the balls. The converse is true if the person is moving away from you. So it is actually the wavelength which is affected; as a consequence, the perceived frequency is also affected.

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If the moving source is emitting waves with an actual frequency f0, then an observer stationary relative to the medium detects waves with a frequency f given by:

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:f = f_0 left ( rac {v}{v + v_{s,r}} ight )

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where v is the speed of the waves in the medium and vs, r is the speed of the source with respect to the medium (negative if moving towards the observer, positive if moving away), radial to the observer.

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A similar analysis for a moving observer and a stationary source yields the observed frequency (the observer's velocity being represented as vo):

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:f = f_0 left (1 + rac {v_0}{v} ight )

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The first attempt to extend Doppler's analysis to light waves was soon made by Fizeau. In fact, light waves do not require a medium to propagate and the correct understanding of the Doppler effect for light requires the use of the Special Theory of Relativity. See relativistic Doppler effect.

Related Topics:
Light - Fizeau - Special Theory of Relativity - Relativistic Doppler effect

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