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| Applications of Doppler Effect |
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 The Doppler effect provides a convenient means of tracking a satellite that is emitting a radio signal of constant frequency. The frequency of the signal received on the Earth changes as the satellite is passing. If the received signal is combined with a constant signal generated in the receiver to give rise to beats, then the beat can have a frequency that produces an audible note, whose pitch decreases as the satellite passes overhead. |
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 The traffic police use a technique based on Doppler principle to detect overspeeding of vehicles on highways. An electromagnetic wave is emitted by a source at the side of the road attached to a police car. The wave is reflected by a moving vehicle, which thus, acts as a moving source. The reflected wave will have a Doppler shift in frequency. Measurement of the frequency shift using the phenomenon of beats, permits the measurement of the speed of the vehicle. |
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 The Doppler effect for light is important in astronomy. Two stars which revolve around one another are called double stars or spectroscopic binaries. When one is approaching the Earth, the other will be receding and this causes a split in the spectral lines, due to change in the frequency of the light emitted. The phenomenon of red shift in the light from stars helps in understanding the theory of expanding universe. |
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