Q. A laser beam can be focused on an area equal to the square of its wavelength, $A\ He$-$Ne$ laser radiates energy at the rate of $1mV$ and its wavelength is $632.8\ nm$. The intensity of focussed beam will be
A) $1.5 \times 10^{13}W/m^{2}$
B) $2.5 \times 10^{9}W/m^{2}$
C) $3.5 \times 10^{17}W/m^{2}$
D) None of these
Explanation
(b) Area through which the energy of beam passes
$= \left( 6.328 \times 10^{- 7} \right)^{2} = 4 \times 10^{- 13}m^{2}$
$\therefore I = \frac{P}{A} = \frac{10^{- 3}}{4 \times 10^{- 13}} = 2.5 \times 10^{9}W/m^{2}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.