First slide
Doppler's effect
Question

A small source of sound moves on a circle as shown in the figure and an observer is standing on O. Let n1, n2 and n3 be the frequencies heard when the source is at A, B and C respectively. Then 

Moderate
Solution

At point A, source is moving away from observer so apparent frequency n1<n  (actual frequency) At point B source is coming towards observer so apparent frequency n2>n and point C source is moving perpendicular to observer so  n3=n

Hence   n2>n3>n1

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