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# A laser emits a beam of light of $2mm$ diameter. If the power of beam is $10mW$, the intensity of the beam of light is

$\begin {array} {1 1} (a)\;31.8 \times 10^3W/m^2 & \quad (b)\;3.18 \times 10^3W/m^2 \\ (c)\;31.8 \times 10^5W/m^2 & \quad (d)\;3.18 \times 10^5W/m^2 \end {array}$

Area of the beam $= A = \large\frac{\pi (diameter)^2}{4}$
$= \large\frac{3.14 \times 4 \times 10^{-6}}{ 4} $$= 3.14 \times 10^{-6}m^2 Intensity = \large\frac{Power}{A} = \large\frac{10 \times 10^{-3}} { 3.14\times10^{-6}}$$ = 3184.713W/m^2 = 3.18 \times 10^3 W/m^2$
Ans : (b)