An increase (or decrease) in the frequency of sound, light, or other waves as the source and observer move towards (or away from) each other. The effect causes the sudden change in pitch noticeable in a passing siren, as well as the red shift seen by astronomers.
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Doppler Effect:
The apparent frequency of sound heard by the observer due to the relative motion between the source and the observer is called Doppler Effect.
- The technique of Doppler effect is, mainly applied by sending a sound wave or an   electromagnetic wave towards a moving object and determining the apparent frequency of   the reflected wave.
Eg: The frequency of sound heard by a person when a train is approaching him.
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