The driver of a car travelling with speed `30ms^-1` towards a hill sounds a horn of frequency 600 Hz. If the velocity of sound in air is `330ms^-1`, the frequency of reflected sound as heard by driver is

Options

A. 555.5 Hz
B. 720 Hz
C. 500 Hz
D. 550 Hz

Answer

B

Explanation

The frequency of the sound that reaches the hill is v'

`v' = ((v + v_(L))/(v - v_(S)))v`
`= ((330 + 0)/(330 - 60)) xx 600`
v' = 660 Hz
After that the sound gets reflected from the hill. In this case hill behaves like a source of the sound and driver behaves as lestener.
Thus, frequency heard by driver

`v' = ((upsilon + upsilon_(L))/(upsilon + upsilon_(S)))v'`
`= ((330 + 30)/(330 + 0)) 660`
v' = 720 Hz