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A siren emitting a sound of frequency 800 Hz moves away from an observer towards a cliff at a speed of 15 m/s. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is (Take velocity of sound in air = 330 m/s):

1. 765 Hz
2. 800 Hz
3. 838 Hz
4. 885 Hz

Correct answer: 765 Hz

Solution

The siren moves away from the observer, so the frequency heard by the observer will be lower due to the Doppler effect. First, the frequency heard by the cliff is calculated using the Doppler formula, and then the reflected sound is treated as a new source moving towards the observer. The final frequency heard by the observer is 765 Hz.

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