CBSE Class 11-science Answered

When a source of sound is approaching or is being approached by an observer, the pitch of the sound appears to be higher than its normal, and when it is receding or being receded by the observer, the pitch of the sound appears to fall below the normal.  This apparent change in the pitch due to the relative motion between the source and the observer (and the medium) is called Doppler's effect.

When a whistling engine approaches an observer who is standing on the platform, he notices a rise in pitch, and fall in pitch when the engine recedes from him.

It can be explained as follows.  When the whistling engine is approaching, the engine is following the waves already sent to the observer.  This results in crowding up of the waves.  The waves reach the observer in quicker succession and the pitch or the frequency, therefore, appears to rise.  The converse happens when the engine is receding from the observer.