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 (a) 1.2 f, 1.2 λ (b) 1.2 f, λ (c) f, 1.2 λ (d) 0.8 f, 0.8 λ

Answer: (b). Observer approaching: f' = f(v + v/5)/(v) = 1.2f. The source is at rest, so the waves in air are not compressed: the wavelength stays λ. So the answer is (b) 1.2f, λ.

An observer moves towards a stationary source of sound with a speed… | NEET 2003 (AIPMT)

Q79NEET 2003 · AIPMTEasy

Doppler Effect

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 $\lambda$ and $f$ respectively. The apparent frequency and wavelength recorded by the observer are respectively

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Answer: (b)

Observer approaching: $f' = f\frac{v + v/5}{v} = 1.2f$ .
The source is at rest, so the waves in air are not compressed: the wavelength stays $\lambda$ .
So the answer is (b) $1.2f$ , $\lambda$ .

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