Question

Speed of sound wave is v. If a reflector moves towards a stationary source emitting waves of frequency f with speed u, the wavelength of reflected waves will be

• A.
• B.
• C.
• D.

Answer 1

The correct option is C

Apparent frequency for reflector (which will act here as an observer) would be $f_1=\left(\frac{v+u}{v}\right)f$ Where f is the actual frequency of source. The reflector will now behave as a source. The apparent frequency will now become
$f_2=\left(\frac{v}{v-u}\right)f_1$
Substituting the value of $f_1$ we get
$f_2=\left(\frac{v+u}{v-u}\right)f$