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
2.5× 109 W/m2
Area through which the energy of beam passes
=(6.328× 10−7)=4× 10−13m2
∴ I=PA=10−34× 10−13=2.5× 109W/m2