Doppler Effect

When a source and an observer separated by a large distance are at rest relative to the medium (usually air) the detector (usually listener) receives the same number of waves per second, as that emitted by the source. However, if there is a motion of source and/or observer relative to the medium along the line of sight, ie, the line joining the source to the observer, the number of waves received per second is usually different from that emitted and the frequency or pitch of source appear to be different. This apparent change in frequency due to motion of source and observer relative to the medium along the line of sight is called Dopplers effect.