Question:
If $2.5 \times 10^{-6} \mathrm{~N}$ average force is exerted by a light wave on a non-reflecting surface of $30 \mathrm{~cm}^{2}$ area during 40 minutes of time span, the energy flux of light just before it falls on the surface is _____________ $\mathrm{W} / \mathrm{cm}^{2}$.
(Round off to the Nearest Integer)
(Assume complete absorption and normal incidence conditions are there)
Solution:
$\mathrm{F}=\frac{\mathrm{IA}}{\mathrm{C}}$
$\mathrm{I}=\frac{\mathrm{FC}}{\mathrm{A}}=\frac{2.5 \times 10^{-6} \times 3 \times 10^{8}}{30}=25 \mathrm{~W} / \mathrm{cm}^{2}$
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