A railway engine whistiling at a constant frequency moves with a constant speed. It goes past a stationary observer beside the railyway track. Which of the following graphs best represent the variation of frequnecy of the sound (n) heard by the observer with the time (t) ?

Options

A.
B.
C.
D.

Answer

B

Explanation

Here the source is moving and the observer is stationary. Also upto time `t=t_(c),` the source is approaching the observer, the frequency of sound of the whistle heard then by the observer is given by
`n_(1)=n*(V)/(V-v)`
and it remains constant upto time `t_(c)` where the railway engine is crossing the observer. after that the source (engine) recedes away from the observer and the frequency will be given by
`n_(2)=(V)/(V+v)*n`
where V is the velocity of sound, v is the velocity of the railway engine and n is the actual frequency of the sound of the whistle. there is a sudden change ini the frequency heard by the observer at `t=t_(c)` and then it remains constant. so there is a discontinuity in the plot of n vs t.