Question

A laser lamp is of $9mW$ diameter $=2mm$. Then what is the amplitude of magnetic field associated with it?

• A. $49\mu$ $T$
• B. $98\mu$ $T$
• C. $9.8\mu$ $T$
• D. $4.9\mu$ $T$

Answer 1

The correct option is D $4.9\mu$ $T$

$S=\frac{P}{A}=c{ϵ}_0{E}_{rms}^2$

$E_{rms}=\sqrt{\frac{P}{Ac{ϵ}_0}}$

$=\sqrt{\frac{0.009}{\pi \times \left({10}^{-3}\right)\times (3\times {10}^8)\times (8.85\times {10}^{12})}}$

$=1038.7V/m$

$B_{rms}=\frac{E_{rms}}{c}=\frac{1038.7}{3\times {10}^8}=3.46\mu T$

$B=\sqrt{2}{B}_{rms}$

$=4.9\mu T$