Question

Sun light falls normally on a surface of area $36\, cm ^2$ 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. $25.92 \times 10^2 \, W / cm ^2$
• B. $8.64 \times 10^{-6} \, W / cm ^2$
• C. $6.0 \, W / cm ^2$
• D. $0.06 \, W / cm ^2$

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 }{ cm ^2}$