Q. A sound wave of frequency $n$ travels horizontally to the right. It is reflected from a large vertical plane surface moving to the left with speed v. The speed of the sound in the medium is c. Then
A) The frequency of the reflected wave is $n\left\lbrack \frac{c + v}{c - v} \right\rbrack$
B) The wavelength of the reflected wave is $\left\lbrack \frac{c}{n} \right\rbrack\left\lbrack \frac{c + v}{c - v} \right\rbrack$
C) The number of waves strike the surface per second is $n\left\lbrack \frac{c + v}{c} \right\rbrack$
D) The number of beats heard by a stationary listener to the left to the reflecting surface is $\left\lbrack \frac{nv}{c - v} \right\rbrack$
Explanation
(c) Large vertical plane acts as listener moving towards source with a velocity v.
$\therefore n^{'} = \frac{(c + v)n}{c}$
This is the number of waves striking the surface per second.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.