Question

An observer moves towards a stationary source of sound with a speed $\frac{1}{5}^{t h}$ of the speed of sound. The wavelength and frequency of the source emitted are $\lambda $ and $f$ respectively. The apparent frequency and wavelength recorded by the observer are respectively

• A. $1.2f,1.2\lambda $
• B. $1.2f,\lambda $
• C. $f,1.2\lambda $
• D. $0.8f,0.8\lambda $

Answer 1

Answer: (B)

Apparent frequency, $f^{'}=\left(\frac{v + v_{0}}{v}\right)f$
$\Rightarrow f^{'}=\left(\frac{v + \left(1 / 5\right) v}{v}\right)f=1.2f$
Wavelength does not change by motion of observer.