Light with an energy flux of  $25\mathrm{x}{10}^4\mathrm{W} {\mathrm{m}}^{-2}$ falls on a perfectly reflecting surface at normal incidence. If the surface area is $15{\mathrm{cm}}^2$, the average force exerted on the surface is 
 

$\text{ Energy flux, }I=25\times {10}^4 \mathrm{W}{ \mathrm{m}}^{-2}$

$\text{ Area, }A=15{ \mathrm{cm}}^2=15\times {10}^{-4}{ \mathrm{m}}^2$

$\text{ Speed of light, }c=3\times {10}^8 \mathrm{m}{ \mathrm{s}}^{-1}$

$For a perfectly reflecting surface, the average force exerted on the surface is$

$F=\frac{2IA}{c}=\frac{2\times 25\times {10}^4{\mathrm{Wm}}^{-2}\times 15\times {10}^{-4}{ \mathrm{m}}^2}{3\times {10}^8 \mathrm{m}{ \mathrm{s}}^{-1}}$

    $=250\times {10}^{-8} \mathrm{N}=2.50\times {10}^{-6} \mathrm{N}$