First slide

Doppler

Question

A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of 15 ${ms}^{- 1}$ . Then. the frequency of sound that the observer hears in the echo reflected from the cliffis:
(Take velocity of sound in air = 330 ${ms}^{- 1}$ )

Moderate

A

885 Hz
B

765 Hz
C

800 Hz
D

838 Hz

Solution

Frequency at the wall will be
$f^{'} = f_{0} (\frac{v - v_{0}}{v - v_{s}}) = 800 (\frac{330 - 0}{330 - 15})$
= $800 (\frac{330}{315}) = 838 Hz$
Since the observer and the wall are stationary so frequency of echo observed by the observer will also be 838 Hz.

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