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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.

The siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer.

Note:   The above text is excerpted from the Wikipedia article "Doppler effect", which has been released under the GNU Free Documentation License.
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July 3, 2015

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