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Doppler effect

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An increase in the frequency (1) of a sound wave, or of an electromagnetic wave such as light, emitted by a source moving towards the observer, or a decrease in frequency from a source moving away from the observer, a familiar example being the rise and fall in the pitch of an ambulance siren as it approaches and then recedes, the effect being the result of a compression or stretching out of waves due to the motion of the source relative to the observer. Also called the Doppler shift. See also biosonar. [Named after the Austrian physicist Christian Johann Doppler (1803–53) who formulated it in 1842]

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