Discussion Forum



1)

An obstacle is moving towards the source with velocity v. The sound is reflected from the obstacle. If c is the speed  of sound and $\lambda$ is the wavelength, then the wavelength of the reflected wave, $\lambda_{r}$is 


A) $\lambda_{r}=\left(\frac{C-V}{C+V}\right)\lambda$

B) $\lambda_{r}=\left(\frac{C+V}{C-V}\right)\lambda$

C) $\lambda_{r}=\left(\frac{C-V}{C}\right)\lambda$

D) $\lambda_{r}=\left(\frac{C+V}{C}\right)\lambda$

Answer:

Option A

Explanation:

 The  frequency of reflected sound wave is 

$f_{r}=f\left(\frac{C+V}{C-V}\right)$

 $\because$ No change  in velocity  occurs due to reflection of sound wave.

Hence,    $\frac{C}{\lambda_{r}}=\frac{C}{\lambda}\left(\frac{C+V}{C-V}\right)\Rightarrow \frac{1}{\lambda_{r}}=\frac{1}{\lambda_{}}\left(\frac{C+V}{C-V}\right)$

$\lambda_{r}=\left(\frac{C-V}{C+V}\right)\lambda$