What is 'Doppler effect' ?

Explanation

Doppler effect `:` The apparent change in the frequency heard by the observer due to relative motion between the oberser and the source of sound is called doppler effect.
Example `:`
(1) The frequency of whistling engine heard by a person standing on the platform appears to increase, when the engine is approaching the platform and it appears to decrease when the engine is moving away from the platform.
(2) Due to Doppler effect the frequency of sound emitted by the siren of an approaching ambulance appears to increase. Similarly the frequency of sound appears to drop when it is moving away.