A sound source $S$ is moving along a straight track with speed $\mathrm{v}$, and is emitting sound of frequency $\mathrm{v}_{0}$ (see figure). An observer is standing at a finite distance, at the point $\mathrm{O}$, from the track. The time variation of frequency heard by the observer is best represented by :
$\left(\mathrm{t}_{0}\right.$ represents the instant when the distance between the source and observer is minimum)
Correct Option: , 4
$\mathrm{f}_{\text {observed }} \Rightarrow\left(\frac{\mathrm{v}_{\text {sound }}}{\mathrm{v}_{\text {sound }}-\mathrm{v} \cos \theta}\right) \mathrm{f}_{0}$
initially $\theta$ will be less $\Rightarrow \cos \theta$ more
$\therefore \mathrm{f}_{\text {observed }}$ more, then it will decrease.
$\therefore$ Ans. (4)
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