Question

A source of sound producing wavelength $50cm$ is moving away from a stationary observer with ${\frac{1}{5}}^{th}$ speed of sound. Then what is the wavelength of sound heard by the observer?

• A. $55cm$
• B. $40cm$
• C. $60cm$
• D. $70cm$

Answer 1

Answer: (C)

$60cm$

Let the speed of sound be $v$. Then speed of the source will be $\frac{v}{5}$. Since the source is moving away from the observer,

$f^{\prime }=f\frac{v}{v-v_s}=f\frac{v}{v+\frac{v}{5}}=\frac{5}{6}f$

So, $\lambda =\frac{v}{f^{\prime }}=\frac{v}{\frac{5}{6}f}=\frac{6}{5}\lambda =\frac{6}{5}\times 50cm=60cm$