Sun light falls normally on a surface of area $36 \mathrm{~cm}^{2}$ and exerts an average force of $7.2 \times 10^{-9} \mathrm{~N}$ within a time period of 20 minutes. Considering a case of complete absorption, the energy flux of incident light is

(D) $0.06 \mathrm{~W} / \mathrm{cm}^{2}$

(A) $25.92 \times 10^{2} \mathrm{~W} / \mathrm{cm}^{2}$

(B) $8.64 \times 10^{-6} \mathrm{~W} / \mathrm{cm}^{2}$

(C) $6.0 \mathrm{~W} / \mathrm{cm}^{2}$

JEE Main · 2022 · 28 Jul 2022 · Shift 2 · EM Waves

Sun light falls normally on a surface of area

36 cm^{2}

and exerts an average force of

7.2 \times 1 0^{- 9} N

within a time period of 20 minutes. Considering a case of complete absorption, the energy flux of incident light is

Answer: (D)

Radiation pressure for complete absorption:

P = \frac{I}{c}

, where

I

is intensity and

c

is speed of light.

Force on the surface:

F = P \times A = \frac{I}{c} \times A

.

Given

F = 7.2 \times 1 0^{- 9}

N and

A = 36 \times 1 0^{- 4}

m²:

\frac{I}{c} \times 36 \times 1 0^{- 4} = 7.2 \times 1 0^{- 9}

I = \frac{7.2 \times 1 0 ^{- 9} \times 3 \times 1 0 ^{8}}{36 \times 1 0 ^{- 4}} = 6 \times 1 0^{2} \frac{W}{m ^{2}} = 0.06 \frac{W}{cm ^{2}}

JEE Main · 2022 · 28 Jul 2022 · Shift 2 · Single correct option