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Question

Find the energy stored in a 60 cm length of a laser beam operating at 4 mW.

A
8×1012J
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B
4×1012J
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C
8×1014J
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D
4×104J
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Solution

The correct option is A 8×1012J
Given L=60 cm=60×102 m
Operating power is given as
P=Ut
U=P×t ....(i)
So we can find t by
t=Lc=60×1023×108=2×109 s
Putting this in eq(i)
U=4×103×2×109=8×1012 J


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