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Light with an energy flux of $25 \times {10^4}$ $W/m^2$ falls on a perfectly reflecting surface at normal incidence. If the surface area is $15\,\, cm^2$ the average force exerted on the surface is
$1.25\times 10^{-6}\;N$
$2.50\times 10^{-6}\;N$
$1.2\times 10^{-6}\;N$
$3 \times 10^{-6}\;N$
Solution
Here, Energy flux, $I=25 \times 10^{4} \,\mathrm{Wm}^{-2}$
Area, $A=15 \,\mathrm{cm}^{2}=15 \times 10^{-4}\, \mathrm{m}^{2}$
Speed of light, $c=3 \times 10^{8} \,\mathrm{ms}^{-1}$
For a perfectly reflecting surface, the average force exerted on the surface is
$F =\frac{2 I A}{c}=\frac{2 \times 25 \times 10^{4} \,\mathrm{Wm}^{-2} \times 15 \times 10^{-4}\, \mathrm{m}^{2}}{3 \times 10^{8}\, \mathrm{m} \mathrm{s}^{-1}}$
$=250 \times 10^{-8}\, \mathrm{N}=2.50 \times 10^{-6}\, \mathrm{N}$
