Question

Light with energy flux of $18{\text{W/cm}}^2$ falls on a non-reflecting surface, of area $20{\text{cm}}^2$, at normal incidence. The momentum delivered in $30\text{minute}$ is -

• A. $1.08\times {10}^{-5}{\text{kg-ms}}^{-1}$
• B. $2.16\times {10}^{-3}{\text{kg-ms}}^{-1}$
• C. $1.08\times {10}^{-3}{\text{kg-ms}}^{-1}$
• D. $2.16\times {10}^{-5}{\text{kg-ms}}^{-1}$

Answer 1

The correct option is B $2.16\times {10}^{-3}{\text{kg-ms}}^{-1}$

Momentum delivered,

$P=\frac{U}{c}=\frac{IAt}{c}$

$\Rightarrow P=\frac{18\times {10}^4\times 20\times {10}^{-4}\times 30\times 60}{3\times {10}^8}$

$\Rightarrow P=2.16\times {10}^{-3}{\text{kg-ms}}^{-1}$

Hence, option $\left(B\right)$ is the correct answer.