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 632.8 nm. The intensity of focussed beam will be:

1.5 × 1013 W/m2 2.5 × 109 W/m2 3.5 × 1017 W/m2 None of these

2.5 × 109 W/m2

Area through which the energy of beam passes = (6.328 × 10-7)2 = 4 × 10-13 m2 ∴ I = $\frac{P}{A}=\frac{10^{-3}}{4 \times 10^{-13}}$ = 2.5 × 109 W/m2