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# Find the average energy stored in a 60 cm length of a laser beam operating at $4\: mW.$

$\begin {array} {1 1} (a)\;0.8 \times 10^{-10} J & \quad (b)\;8 \times 10^{-10} J \\ (c)\;0.8 \times 10^{-12} J & \quad (d)\;8 \times 10^{-12} J \end {array}$

Time, $t = \large\frac{60 \times 10^{-2}}{ (3 \times 10^8)}$$= 2 \times 10^{-9}s$
So,$U = 4\: mW \times 2 \times 10^{-9}s = 8 \times 10^{-12} J$
Ans : (d)