Waves

A source of sound emits sound waves at frequency \(f_0\). It is moving towards an observer with fixed speed \({v_s}\) (\({v_s},v\), where \(v\) is the speed of sound in air.) If the observers were to move towards the source with speed \(v_0\), one of the following two graphs (A and B) will give the correct variation of the frequency \(f\) heard by the observer as \(v_0\) is changed.

The variation of \(f\) with \(v_0\) is given correctly by