A 27 mW laser beam has a cross-sectional maximum electric field in this electromagnetic wave is given by :
[Given permittivity of space `in_(0) = 9 xx 10^(12)` SI units, Speed of light `c= 3 xx 10^(8) m//s`]

Options

A. 1 kV/m
B. 0.7 kV/m
C. 2 kV/m
D. 1.4 kV/m

Answer

D

Explanation

`I=1/2ncin_(0)E^(2)`
Where, n is refractive index, for air n=1
`I=1/2c.epsilon_(0)E^(2)` …(i)
Also, `I=P/A` …(ii)
From Eq. (i) and (ii), we get
`1/2c.epsilon_(0)E^(2)=P/A` or `1/2cepsilon_(0)E^(2)=P/A` or `E=sqrt((2xx27xx10^(-3))/(10xx10^(-6)xx3xx10^(8)xx9xx10^(-12)))`
`=1.4xx10^(3)V//m=1.4kV//m`