A train whistling at constant frequency is moving towards a station at a constant speed V. The train goes past a stationary observer on the station. The frequency `n^(')` of the sound as heard by the observer is plotted as a function of time t, figure. Identify the expected curve.

Options

A.
B.
C.
D.

Answer

C

Explanation

When train is approaching a stationary observer, the apparent frequency observed is given by
`n'= (upsilon - upsilon_(0))/(upsilon - upsilon_(s)) n = ((upsilon)/(upsilon - V))n` = constant
Also, `n' gt n`
After crossing the stationary observer, the apparent frequency heard by observer is given by
`n' = (upsilon-upsilon_(0))/(upsilon - upsilon_(s)) n = ((upsilon)/(upsilon + V))n` = constant
Here `n' lt n`
The correct option is (c).