Question

If $2.5\times {10}^{-6}\text{N}$ average force is exerted by a light wave on a non-reflecting surface of area $30{\text{cm}}^2$ during $40$ minutes of time span, the energy flux of light just before it falls on the surface is ______ ${\text{W/cm}}^2$.

$($Round off to the nearest integer$)$
$($Assume complete absorption and normal incidence conditions are there$)$

Answer 1

We know that :
Radiation pressure $=\frac{\text{Intensity}}{\text{speed of light}}$
(for absorbing surface)

$\Rightarrow I=P\times c$

$\Rightarrow I=\frac{2.5\times {10}^{-6}}{30\times {10}^{-4}}\times 3\times {10}^8$

$\Rightarrow I=250000{\text{W/m}}^2=25{\text{W/cm}}^2$