Question

A source of sound produces waves of $\lambda =60$cm in air. It is moving with a velocity one-sixth the velocity of sound towards a stationary observer. The apparent wavelength noted by the observer is?

• A. $30$ cm
• B. $60$ cm
• C. $50$ cm
• D. $75$ cm

Answer 1

The correct option is C $50$ cm

The frequency observed by the observer is-

$f=\frac{v}{v-u}{f}_o=\frac{v}{v-\frac{v}{6}}{f}_o=\frac{6}{5}{f}_o$

$\lambda =\frac{v}{f}$

$⟹f\propto \frac{1}{f}$

$\frac{\lambda }{{\lambda }_o}=\frac{f_o}{f}=\frac{5}{6}$

$⟹\lambda =\frac{5}{6}\times 60$

$⟹\lambda =50cm$

Answer-(C)