Question

A laser beam can be focussed on an area equal to the square of its wavelength. A He-Ne laser radiates energy at the rate of 1mW 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

Answer 1

The correct option is B

$2.5\times {10}^{9}W/m^2$

Area through which the energy of beam passes
$=(6.328\times {10}^{-7})=4\times {10}^{-13}{m}^2$

$\therefore I=\frac{P}{A}=\frac{{10}^{-3}}{4\times {10}^{-13}}=2.5\times {10}^{9}W/m^2$