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  4. A sound wave of frequency f propagating through the air with a velocity c , is reflected from a wall which is moving away from the stationary source with a constant speed v . The frequency of the reflected wave, measured by the observer at the position of the source is

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Q. A sound wave of frequency $f$ propagating through the air with a velocity $c$ , is reflected from a wall which is moving away from the stationary source with a constant speed $v$ . The frequency of the reflected wave, measured by the observer at the position of the source is

NTA AbhyasNTA Abhyas 2020Waves

Solution:

In first case observer (surface) is moving away. perceiver frequency
$f^{'} = f \left(\right. \frac{c - v}{c} \left.\right)$
In second case source (surface) is moving away. Perceived frequency
$f^{''} = f^{'} \left(\right. \frac{c}{c + v} \left.\right) = f \left(\right. \frac{c - v}{c + v} \left.\right)$