Question

A whistle sends out 256 waves in a second. If the whistle approaches the observer with velocity 1/3 of the velocity of sound in air, the number of waves per second the observer will receive

• A. 384
• B. 192
• C. 300
• D. 200

Answer 1

The correct option is A

384

Wave number = $\frac{1}{\lambda }but\frac{1}{{\lambda }^{\prime }}=\frac{1}{\lambda }\left(\frac{v}{v-v_s}\right)and{v}_s=\frac{v}{3}$

$\therefore$ ( W.N.)' = ( W.N.) $\left(\frac{v}{v-\frac{v}{3}}\right)=256\times \frac{v}{\frac{2v}{3}}$

= $\frac{3}{2}\times 256=384$