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When a source generating waves moves relative to an observer, or whenan observer moves relative to a source, there is an apparent shift infrequency. If the distance between the observer and the source isincreasing, the frequency apparently decreases, whereas the frequencyapparently increases if the distance between the observer and thesource is decreasing. This relationship is called Doppler Effect (orDoppler Shift) after Austrian Physicist Christian Johann Doppler (1803-1853).

The relationship describing the Doppler Shift for a moving source is given by:f₂ = f₁v / (v ± v_s)
where f₂ is the apparent frequency, f₁is the actual frequency emitted by the source, v is the speed of sound in the medium, v_s is the speed of the source through the medium (the negative sign is usedif the source is moving towards the observer).

The relationship describing the Doppler Shift for a moving observer is given by:f_o = f_s(v ± v_o) / v
where f_o is the observed frequency, f_s is thesource frequency, v is the speed of sound, v_o is the speedof the observer (it is taken to be negative if the observer is receding from the source).
The Doppler Effect explains the apparent change in pitch ofa passing automobile. Of course, the frequency of the sound emitted by asource remains unchanged, and so does the velocity of the sound in thetransmitting medium. A similar effect (Doppler Shift for light) can also beused to determine the speed of a star relative to the earth. The red shift ofthe star's spectrum indicates that the distance between an observed star andthe earth may be increasing. The Doppler Shift for light describes a changein wavelength, not a change in frequency as with sound. Short range radardevices use the Doppler Shift principle. A change in frequency betweenemitted and returning pulses can be used to find the relative speed.