Doppler effect

Doppler effect, the apparent difference between the 
frequency at which sound or light waves leave a source and that 
at which they reach an observer, caused by relative motion of 
the observer and the 
wave source. This phenomenon is used in astronomical measurements, 
in Moussbauer effect studies, and in radar and modern navigation. 
It was first described (1842) by Austrian physicist Christian 
Doppler.
 The following is an example of the Doppler effect: as one approaches 
a blowing horn, the perceived pitch is higher until the horn 
is reached and then becomes lower as the horn is passed. Similarly, 
the light from a star, observed from the Earth, shifts toward 
the red end of the spectrum (lower frequency or longer wavelength) 
if the Earth and star are receding from each other and toward 
the violet (higher frequency or shorter wavelength) if they 
are approaching each other. The Doppler effect is used in studying 
the motion of stars and to search for double stars and is an 
integral part of modern theories of the universe. See also 

red shift

Related Propaedia Topics:

General properties of waves: frequency, amplitude, wavelength, phase

The velocity of waves

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