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Question

A star moving away from earth with a speed of 0.02c , while emitting light of frequency 6×1014 Hz. What frequency will be observed on earth?
(Take c=3×108 m/s)

A
0.24×1014 Hz
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B
1.2×1014 Hz
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C
5.88×1014 Hz
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D
0.12×1014 Hz
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Solution

The correct option is C 5.88×1014 Hz
The change in frequency is given by,
Δff=vrc Δf=f×vrc

Δf=6×1014×0.02 cc=0.12×1014 Hz

As the star is moving away from earth, the apparent frequency will be less than actual frequency, i.e. f<f

f=fΔf=6×10140.12×1014
=5.88×1014 Hz

Hence, (B) is the correct answer.

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