A whistle sends out $256$ waves in a second. If the whistle approaches the observer with velocity $\frac{1}{3}$ of the velocity of sound in air, the number of waves per second the observer will receive
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(a) Wave number = $\frac{1}{\lambda }$ but $\frac{1}{{\lambda '}} = \frac{1}{\lambda }\left( {\frac{v}{{v - {v_s}}}} \right)\,{\rm{and}}\;{v_s} = \frac{v}{3}$
$\therefore $ $(W.N.)’ = (W.N.)$ $\left( {\frac{v}{{v - v/3}}} \right) = 256 \times \frac{v}{{2v/3}}$
= $\frac{3}{2} \times 256 = 384$
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