(i) Find the energy stored in a 90 cm length of a laser beam operating at `6 mW`.
(ii) Find the amplitude opf electric field in a parallel beam of light of intensity `17.7 W//m^(2)`.

Explanation

(i) The time taken by wave to move a distance `90 cm`.
`t = (90 xx 10^(-2))/(3 xx 10^(8)) = 3 xx 10^(-9)s`
Energy contained in `90` cm length,
`u = Pt = 6 xx 10^(-3) xx 3 xx 10^(-9) = 18 xx 10^(-12)J`
(ii) Intensity of light, `I = (1)/(2) epsi_(0)E_(0)^(2)c`
`rArr 17.7 = 1/2(8.85 xx 10^(-12))E_(0)^(2) xx 3 xx 10^(8)`
`rArr E_(0)^(2) = 4/3 xx 10^(4)`
Therefore the amplitude of the electric field in the parallel beam, `E_(0) = 2/3 xx 10^(2)V//m`