First slide

Quantum Theory of Light

Question

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

Easy

A

$1.25 \times 10^{- 6} N$
B

$2.50 \times 10^{- 6} N$
C

$1.20 \times 10^{- 6} N$
D

$3.0 \times 10^{- 6} N$

Solution

$Energy flux, I = 25 \times 10^{4} W m^{- 2}$
$Area, A = 15 {cm}^{2} = 15 \times 10^{- 4} m^{2}$
$Speed of light, c = 3 \times 10^{8} m s^{- 1}$
$F o r a p e r f e c t l y r e f l e c t i n g s u r f a c e, t h e a v e r a g e f o r c e e x e r t e d o n t h e s u r f a c e i s$
$F = \frac{2 I A}{c} = \frac{2 \times 25 \times 10^{4} {Wm}^{- 2} \times 15 \times 10^{- 4} m^{2}}{3 \times 10^{8} m s^{- 1}}$
$= 250 \times 10^{- 8} N = 2.50 \times 10^{- 6} N$

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