Explain what is meant by Doppler’s effect. – NABTEB QUESTION

Doppler’s effect can be defined as the change in the frequency of sound heard by an observer due to the relative motion of the source and the observer. An example is the roar of the motor of a racing car as it approaches and recedes from an observer at the race track. The motor seems to slow down as it passes the observer. Or also the pitch of the whistle on a fast moving train sounds higher as the train approaches the observer at a station than it does as the train passes by