Question

Light with an energy flux of 18W $cm^{- 2}$falls on a non-reflecting surface at normal incidence. If the surface has an area of 20$cm^{2}$, the average force exerted on the surface during a 30 minute time span is :

• A. $2.1 \times 10^{- 6}N$
• B. $1.2 \times 10^{- 6}N$
• C. $2.1 \times 10^{6}N$
• D. $1.2 \times 10^{6}N$

Answer 1

Answer: (B)

$1.2 \times 10^{- 6}N$

Answer 2

: The total energy falling on the surface is

$U = 18 \times 20 \times (30 \times 60) = 6.48 \times 10^{5}J$

Therefore, the total momentum delivered

(for complete absorption is)

$P = \frac{U}{c} = \frac{6.48 \times 10^{5}}{3 \times 10^{8}} = 2.16 \times 10^{- 3}kgms^{- 1}$

The average force exerted on the surface is

$F = \frac{p}{t} = \frac{2.16 \times 10^{- 3}}{0.18 \times 10^{4}} = 1.2 \times 10^{- 6}N$