Question

An observer approaches towards a stationary source of sound at constant velocity and recedes away at the same speed. The graph of wavelength observed with time is:

• A.
• B.
• C.
• D.

Answer 1

The correct option is B



If the source is stationary and observer is moving, then all of the spheres representing the air compressions in the sound wave are centered on the same point, and observer can hear sound of hear the same wavelength. However the frequency heard by him will be higher than actual frequency when he approaches and lower than actual frequency when he recedes away from source.

$\begin{array}{c}\text{Why this question?}\\ \text{Due to the relative movement between the source and the detector}\\ \text{there is an apparent change in the sound frequency but at a constant}\\ \text{velocity this apparent frequency remains constant.}\end{array}$