A sound source $S$ is moving along a straight track with speed $v,$ and is emitting sound of frequency $v_{o}$ (see figure). An observer is standing at a finite clistance, at the point $O$, from the track. The time variation of frequency heard by the observer is best represented by
$\left(t_{0}\right.$ represents the instant when the distance between the source and observer is minimum)
JEE MAIN 2020, Diffcult
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$f_{\text {observed }} \Rightarrow\left(\frac{v_{\text {sound }}}{v_{\text {sound }}-v \cos \theta}\right) f_{0}$
initially $\theta$ will be less $\Rightarrow \cos \theta$ more
$\therefore f _{\text {observed }}$ more, then it will decrease.
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