A driver in a stationary car blows a horn which produces monochromatic sound waves of freqeuncy 1000 Hz normally towards a reflecting wall. The wall approaches the car with a speed of `3.3(m)/(s)`.

Options

A. the frequency of sound reflected from wall and heard by the driver is `1000Hz`
B. the frequency of sound reflected from wall and heard by the driver is `980Hz`
C. the percentage increase in frequency of sound after reflection from wall is `2%`
D. the percentage decrease in frequency of sound after reflection from wall is `2%`

Answer

C

Explanation

The relfected sound appears to be coming from mirror image which appears to move towards driver with velocity double the velocity of wall.
`:.V_(C )=V_(omega)=6.6m//s`
`n_("reflected")=n[(V)/(V-V_(s))]n_("reflected") gt n :.(n_(r ))/(n)=(V)/(V-V_(s))`
`implies(n_(r )-n)/(n)xx100%=(V_(s))/(V-V_(s))xx100%`