Question

78. An observer moves towards a stationary source of sound with a speed 1/5th of the speed of sound. The wavelength and frequency of the source emitted are λ and f respectively. The apparent frequency and wavelength recorded by the observer are respectively :​

Answer 1

Explanation:

Speed of the sound = 1/5th (Given)

Frequency of the source emitted = f (Given)

Wavelength of the source emitted = A (Given)

When both the source of sound and the listener are in motion which relative to each other, then the frequency of the sound heard by the listener is not the same as the source frequency. This is known as the doppler effect.

Thus, according to the doppler effect -

n' = v+v0/v.n

= v+v/5/v.f

= 6/5f

=1.2f

Since the source is stationary, thus the wave length will remain unchanged for the observer. Thus, the apparent frequency and wavelength recorded by the observer are respectively 1.2f

hope it helps you.

Answer 2

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