Question

Light with an energy flux of $25\times {10}^{4}W{m}^{-2}$ falls on a perfectly reflecting surface at normal incidence. If the surface area is $15c{m}^2$, the average force exerted on the surface is

• A. $1.25\times {10}^{-6}N$
• B. $2.50\times {10}^{-6}N$
• C. $1.20\times {10}^{-6}N$
• D. $3.0\times {10}^{-6}N$

Answer 1

The correct option is B $2.50\times {10}^{-6}N$

$F_{av}=\frac{2IA}{c}=\frac{2\times 25\times {10}^4\times 15\times {10}^{-4}}{3\times {10}^8}=250\times {10}^{-8}N=2.5\times {10}^{-6}N$