Question

A sound wave of frequency f travels horizontally to the right. It is reflected from a large vertical plane surface moving to left with a speed v. The speed of sound in medium is c. Then

• A. the number of wave striking the surface per second is $f\frac{(c+v)}{c}$
• B. the wavelength of reflected wave is $\frac{c(c+v)}{f(c+v)}$
• C. The frequency of the reflected wave is $f\frac{(c-v)}{(c-v)}$
• D. the number of beats heard by a stationary listener to the left of the reflecting surface is $\frac{vf}{c-v}$

Answer 1

The correct option is A the number of wave striking the surface per second is $f\frac{(c+v)}{c}$

From Doppler's effect, the frequency of the sound wave as observed by the vertical plane moving towards the source=$f\left(\frac{c-(-v)}{c}\right)=f\left(\frac{c+v}{c}\right)$

This frequency is nothing but the number of waves striking the surface per second itself.