Question

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

• A. $25.92\times 10^{2}W/c{m}^2$
• B. $8.64\times 10^{-6}W/c{m}^2$
• C. $6.0W/c{m}^2$
• D. $0.06W/c{m}^2$

Answer 1

Answer: (D)

$\frac{I}{C}\times \text{ area }=\text{ force }$
$\frac{I}{C}\times 36\times 10^{-4}=7.2\times 10^{-9}$
$I=\frac{7.2\times 10^{-9}\times 3\times 10^8}{36\times 10^{-9}\times 10}$
$=\frac{6\times 10^{-1}}{10^{-3}}$
$I=6\times 10^2\frac{w}{m^2}$
$=0.06\frac{w}{c{m}^2}$