Question

A $27\, mW$ laser beam has a cross-sectional area of $10 \; mm^2$. The magnitude of the maximum electric field in this electromagnetic wave is given by [Given permittivity of space $\in_0 = 9 \times 10^{-12}$ SI units, Speed of light $c = 3 \times 10^8 \; m/s$] :

• A. $1 \,kV/m$
• B. $2 \,kV/m$
• C. $1.4 \,kV/m$
• D. $0.7 \,kV/m$

Answer 1

Answer: (C)

$1.4 \,kV/m$

Answer 2

The correct answer is C:$1.4kv/m$
EM wave's intensity;
$I = \frac{\text{Power}}{\text{Area}} = \frac{1}{2} \varepsilon_{0} E_{0}^{2} C$
Now using this formula and putting in the values we get;
$\frac{10^{-3}\times27}{10^{-6}\times 10}=\frac{1}{2}\times10^8\times E^2\times 10^{-12}\times 3\times 9$
Simplifying above equation we get;
$E=1.4kv/m$