Question

A laser beam 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 $4.9\mu T$

Given,

$P=9mW=0.009W$

$d=2mm$

$r=1mm$

$c=3\times {10}^{8}m/s$

RMS value of electric field will be

$E_{rms}=\sqrt{\frac{P}{\pi r^{2}c{\epsilon }_0}}$

$E_{rms}=\sqrt{\frac{0.009}{3.14\times ({10}^{-3}{)}^2\times 3\times {10}^8\times 8.85\times {10}^{-12}}}=1038.7V/m$

RMS value of magnetic field will be

$B_{rms}=\frac{E_{rms}}{c}$

$B^{}=\frac{1038.7}{3\times {10}^8}=3.46\mu T$

$B=B_{rms}\sqrt{2}=3.46\mu T\times \sqrt{2}$

$B=4.9\mu T$

The correct option is D.