Topic: Wave motion: longitudinal and transverse waves, wave speed· Oscillations and Waves

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 cliff is : (Take velocity of sound in air = 330\ ms^…

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 cliff is : (Take velocity of sound in air \(= 330\ ms^{-1}\))

1. 765 Hz

2. 800 Hz

3. 838 Hz

4. 885 Hz

Doubtly Asked 30 Oct 2026 NEET UG 2016 Phase 1 neet Save WhatsApp Report

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Answer: 3. 838 Hz

Key idea: the cliff is a stationary reflector. The sound reaches it with a Doppler shift because the siren moves toward the cliff, and the cliff re-emits that same frequency to the observer.

The siren moves toward the cliff at \(15\) m/s, so the frequency received by the cliff is \(f'=f\dfrac{v}{v-v_s}=800\times\dfrac{330}{315}\).
\(f'\approx 838\) Hz.
The cliff is at rest, so the observer hears the echo at \(838\) Hz.

Doubtly AI · 30 Oct 2026 AI-generated · not yet verified Report

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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\ ms^-1. Then, the frequency of sound that the observer hears in the echo reflected from the cliff is : (Take velocity of sound in air = 330\ ms^…

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