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

Whistling train is the source of sound, `upsilon_(s)=B`. Before crossing a stationary observer on station, frequency heard is `n^(')=(upsilonn)/((upsilon-upsilon_(s)))=(upsilonn)/(upsilon-V)=` constant, and `n^(')gtn.`
Here , `upsilon` is velocity of sound in air and n is actual frequency of whistle.
After crossing the stationary observer, frequency heard is `n^(')=(upsilon)/((upsilon+upsilon_(s)))=(upsilon)/(upsilon+V)=` constant and `n^(')ltn`
Therefore, the expected curve is (c).