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 $1nW$ and its wavelength is $632.8nm$.The intensity of foucussed beam will be

• A. $1.5\times {10}^{13}W/m^2$
• B. $0.25\times {10}^{4}W/m^2$
• C. $3.5\times {10}^{17}W/m^2$
• D. None of these

Answer 1

The correct option is B $0.25\times {10}^{4}W/m^2$

A laser beam can be focused on an area equal to the square of its wavelength.

A He-Ne laser radial co energy

at the rate$=1nW$

wavelength$=\mathrm{6.32.8}nm$

The intensity of focused beam willbe

Area through which energy of beam passes

$=(6.328\times {10}^{-7})=4\times {10}^{-13}{m}^{2}I=\frac{P}{A}=\frac{{10}^{-9}}{4\times {10}^{-13}}=0.25\times {10}^{4}W/m^2$