Question

If $2.5\times 10^{-6}N$ average force is exerted by a light wave on a non-reflecting surface of $30c{m}^2$ area during $40$ minutes of time span, the energy flux of light just before it falls on the surface is _______ $W/c{m}^2$ (Round off to the Nearest Integer) (Assume complete absorption and normal incidence conditions are there)

Answer 1

Answer: 25

$F=\frac{IA}{C}$
$I=\frac{FC}{A}=\frac{2.5\times 10^{-6}\times 3\times 10^8}{30}=25W/c{m}^2$