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 $ 15 \,c m^2 $, the average force exerted on the surface is

• A. $1.25 \times 10^{-6} N $
• B. $2.50\times10^{-6} N$
• C. $1.20\times10^{-6}N$
• D. $3.0\times10^{-6}N$

Answer 1

Answer: (B)

Here,
Energy flux, $I=25 \times 10^{4} \, W m^{-2}$
Area, $A=15 cm ^{2}=15 \times 10^{-4}\, m ^{2}$
Speed of light, $c=3 \times 10^{8}\, ms ^{-1}$
For a perfectly reflecting surface, the average force exerted on the surface is
$F=\frac{2 I A}{c}=\frac{2 \times 25 \times 10^{4} \,Wm ^{-2} \times 15 \times 10^{-4}\, m ^{2}}{3 \times 10^{8} \,m s ^{-1}}$
$=250 \times 10^{-8} N=2.50 \times 10^{-6} \,N$