**Doppler's Effect: **"The phenomenon of variation in the pitch of a sound due to the relative motion of the source and the observer (listener) is called the Doppler's effect." Due to this effect, the pitch emitted by the siren of an approaching ambulance appears increased. Similarly, the pitch appears to drop when it is moving away.

**The source is moving away from the stationary observer:**

Suppose v is the velocity of a sound in the air and f is the frequency. When both the source and the observer are at rest, the distance occupied by f waves is v per second. Suppose source moves with velocity u_{s} away from the stationary observer O. Therefore, distance occupied by f waves send out towards O in one second becomes v + u_{s}. Thus the wavelength increases as

\(\lambda'=\frac{v+u_s}f\)

The apparent frequency is given by,

\(f'=\frac{velocity\;of\;Sound\;Wave\;relative\;to\;O}{wavelength\;of\;Sound\;reaching\;to\;O}\)

Or, \(f'=\frac v{\lambda'}=\frac v{\displaystyle\frac{v+u_s}f}=\frac v{v+u_s}f\)

Since, \(\left(v+u_s\right)>v\Rightarrow f'<f\).

Thus, the pitch of the sound decreases if the source is moving away from a stationary observer.