First slide

Doppler's effect

Question

A source producing sound of frequency 170 Hz is approaching a stationary observer with a velocity 17 ms^–1. The apparent change in the wavelength of sound heard by the observer is (speed of sound in air = 340 ms^–1)

Moderate

A

0.1m
B

0.2m
C

0.4m
D

0.5m

Solution

$λ = \frac{v}{n} = \frac{340}{170} = 2 m, n' = \frac{340}{340 - 17} \times 170 \Rightarrow n' = 178 .9 Hz$
Now $λ^{'} = \frac{v}{n^{'}} = \frac{340}{178 .9} = 1 .9 m$
$\Rightarrow λ - λ^{'} = 2 - 1 .9 = 0 .1 m$

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