Question

Light with an energy flux of 20 W/$c{m}^2$ falls on a non-reflecting surface at normal incidence. If the surface has an area of 30 $c{m}^2$, the total momentum delivered (for complete absorption) during 30 minutes is :

• A. $3.6\times {10}^{-3}kgm/s$
• B. $3.3\times {10}^{-8}kgm/s$
• C. $10.8\times {10}^{4}kgm/s$
• D. $1.08\times {10}^{7}kgm/s$

Answer 1

The correct option is A $3.6\times {10}^{-3}kgm/s$

Total energy fallen $=$ Power $\times$ time

$=20\times 30\times 30\times 60=1.08\times {10}^{6}J$

$E=\frac{hC}{\lambda }\lambda =\frac{hC}{E}as\lambda =\frac{h}{P}$

Going both equations

$\frac{hC}{E}=\frac{h}{P}$

$P=\frac{E}{C}=\frac{1.08\times {10}^6}{3\times {10}^8}=3.6\times {10}^{-3}kgm/s$