Christian Andreas Doppler
Doppler RADAR is named after Christian Andreas Doppler. Doppler was an Austrian physicist who first described in 1842, how the observed frequency of light and sound waves was affected by the relative motion of the source and the detector. This phenomenon became known as the Doppler effect.
This is most often demonstrated by the change in the sound wave of a passing train. The sound of the train whistle will become "higher" in pitch as it approaches and "lower" in pitch as it moves away. This is explained as follows: the number of sound waves reaching the ear in a given amount of time (this is called the frequency) determines the tone, or pitch, perceived. The tone remains the same as long as you are not moving. As the train moves closer to you the number of sound waves reaching your ear in a given amount of time increases. Thus, the pitch increases. As the train moves away from you the opposite happens.
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