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 1nW and its wavelength is 632.8 nm. The intensity of focussed beam will be:
Answer & explanation
Correct answer: option 2
$\text{Area through which the energy of beam passes is }A = (6.328\times 10^{-7})^2 = 4\times 10^{-13}m^2$
$I = \frac{P}{A} = \frac{10^{-3}}{4\times 10^{-13}} = 2.5\times 10^9 W/m^2$