(a) Find the energy stored in a `90 cm` length of a laser beam operating at `5mW`.
(b) Find the amplitude of the electric field in a parallel beam of light of intensity `8.85 W..m^(2)`.

Explanation

(a) The time taken by wave to move a distance `90 cm`
`t = (10 xx 10^(-2))/(3xx10^(8)) = 3xx10^(-9) sec`
Energy contained in `90 cm` length
`U = Pt = 5 xx 10^(-3) xx 10^(-9) = 15 xx 10^(-12)J`
(b) `I = (1)/(2)in_(0)E_(0)^(2)c`
`0.85 = (1)/(2)(8.85 xx 10(-12))E_(0)^(2) xx 3xx10^(8)`
`E_(0)^(2) = (10^(4))/(1.5)`
`E_(0) = (100)/(sqrt(1.5)) V//m`