Find the energy stored in a 60 cm length of a laser beam operating at 4 mW.
Text Solution
Verified by ExpertsThe correct answer is:
A
The time taken by the electaromagnetic wave to move through a distnce of 60 cm is t = 
= 2 × 10 –9 s.The energy contained in the 60 cm length passes through a cross-section of the beam in 2×10 –9 s. But the energy passing through any cross-section in 2 ×10 –9 s is
U = (4 mW) × (2 × 10 –9 s)
= (4 × 10 –3 J/s) × (2 × 10 –9 s) = 8 × 10 –12 J.
This is the energy contained in 60 cm length.
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