The average force exerted on $5 cm^2$ area of a non-reflecting plate in 10 minutes when light with an energy flux of $20\,W\,cm^{-2}$ incidents normally on it is}
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For an absorbing surface,
\[
F=\frac{P}{c}
\]
and for a perfectly reflecting surface,
\[
F=\frac{2P}{c}
\]