The energy stored in a 90 cm length of a laser beam operating at 10 mW is  $\_\_\_\_\_\_\_\_\_\times {10}^{-12}J$

Time taken by laser beam to move through a distance 90 cm is, $t=\frac{90}{C}$  
$$\begin{gathered} t=\frac{90cm}{3\times {10}^{10}cm/s} \\ \Rightarrow t=30\times {10}^{-10}s \end{gathered}$$ 
The energy contained in $90cm$length of laser beam is 
 $U=power\times time=10\times {10}^{-3}\times 3\times {10}^{-9} J$
 
So,  $U=30\times {10}^{-12}J$