Doppler Effect

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Doppler Effect

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The change in frequency with which energy reaches a receiver when the receiver and the energy source are in motion relative to each other. Also called Doppler shift. </dd>
In the case of sound, or any other wave motion where a real medium of propagation exists (excepting, therefore, light and other electromagnetic radiations) one must distinguish two principal cases: If the source is in motion with speed v relative to a medium which propagates the waves in question at speed c, then the resting observer receives waves emitted, with actual frequency f as if they had a frequency f' given by the Doppler equation

f ' = f/[1 � (v/c)] where the positive sign refers to the

case of the source receding from the observer, and the vice versa for the negative sign. If, on the other hand, the source is at rest relative to the propagating medium while the observer moves with speed v relative to the source,

f ' = f '[1� (v/c)] where the positive sign now refers to the

case of observer approaching the source. For electromagnetic radiation, f/f ' = [1Missing Image:img src="SP7-d_files/minusplus.gif" (v/c)]/[1 � (v/c)] where the top signs represent the source receding from the observer and the bottom signs, approaching the observer. </dd>

References

This article is based on NASA's Dictionary of Technical Terms for Aerospace Use