Question

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

• A. $25.92×10^2 W/cm^2$
• B. $8.64 × 10^{-6 }W/cm^2$
• C. $6.0 W/cm^2$
• D. $0.06 W/cm^2$

Answer 1

Answer: (D)

$0.06 W/cm^2$

Answer 2

$\frac{I}{c}× area = force$

$\frac{I}{c} × 36 × 10^{-4}= 7.2 × 10^{-9}$

$I = \frac{7.2 × 10^{-9} × 3 × 10^8}{36 × 10^{-4}}= \frac{6 × 10^{-1}}{10^{-3}}$

$I = 6 × 10^2 \frac{W}{m^2}= 0.06\frac{W}{cm^2}$