Question

(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).

Answer 1

Given :

For 1st part

1) Length of laser beam = 90 cm

2) Power = 6 mW

For 2nd part

1) Intensity of light = 17.7 W/m^2

To find :

i) Energy stored in laser beam

ii) Amplitude of electric field in parallel beam of light

Solution :

i) Time taken by laser beam to travel 90 cm length is

t = 90/c = 90 / 3×10^8 = 3×10^-9 s

Energy stored in 90 cm length is given by

E = power×time

= (10 mW) × (3×10^9 s)

E = 3×10^-12 J

ii) Intensity of EM waves is given by formula

I = Uav×c = 1/2×εo×Eo^2×c

Eo = [ 2I / εo×c ]^1/2

Eo = [ 2×17.7 / 8.85×10^-12×3×10^8 ]^1/2

= √ 1.288×10^4

= 1.134×10^2

Eo = 113.4 N/C