Find the energy stored in a 60cm length of a laser beam operating at 4mW.
A
8×10−12J
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B
4×10−12J
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C
8×10−14J
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D
4×10−4J
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Solution
The correct option is A8×10−12J Given L=60cm=60×10−2m
Operating power is given as P=Ut ⇒U=P×t ....(i)
So we can find ′t′ by t=Lc=60×10−23×108=2×10−9s
Putting this in eq(i) U=4×10−3×2×10−9=8×10−12J