Question

A source of sound produces waves of wave length 48 cm. This source is moving towards north with speed 1/4 th that of sound. The apparent wavelength of the waves to an observer standing south of the moving source will be

• A. 60 cm
• B. 72 cm
• C. 48 cm
• D. 96 cm

Answer 1

The correct option is A 60 cm

Apparent frequency, $f^{\prime }=\left(\frac{v}{v+v_s}\right)f$
$\Rightarrow f\propto \frac{1}{\lambda }$

$\Rightarrow {\lambda }^{\prime }=\left(\frac{v+v_s}{v}\right)f=\left(\frac{v+\frac{v}{4}}{v}\right)48$ $=\frac{5}{4}\times 48=60cm$