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Question:

If $2.5 \times 10^{-6} \mathrm{~N}$ average force is exerted by a light wave on a nonreflecting 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:

(25)

$F=\frac{I A}{C}$

$I=\frac{F C}{A}=\frac{2.5 \times 10^{-6} \times 3 \times 10^{8}}{30}=25 \mathrm{~W} / \mathrm{cm}^{2}$