Light with an energy flux of 18 W cm-2 falls on a non-reflecting surface at normal incidence. If the surface has an area of 20 cm2, the total momentum delivered to the surface during a span of 30 minutes is:
Answer & explanation
Correct answer: option 1
Total energy falling on the surface = Energy flux × time × area
$\Rightarrow \mathrm{U}=\left(18 \mathrm{~W} \mathrm{~cm}^{-2}\right) ×(30 × 60 s) ×\left(20 \mathrm{~cm}^{-2}\right)=6.48 × 10^5 J$
Total momentum delivered to the surface $\mathrm{p}=\frac{\mathrm{U}}{\mathrm{c}}=\frac{6.48 × 10^5}{3 × 10^8}=2.16 × 10^{-3} kg ~ms^{-1}$