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

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

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Solution:

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