Light with an energy flux of \( 18\,\mathrm{W/cm^2} \), falls at normal incidence on a non-reflecting surface with area of \( 20\,\mathrm{cm^2} \). If the light falls for 30 minutes on the surface, find the total momentum delivered to the surface.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( 2.16\times10^{-3}\,\mathrm{Kg\,ms^{-1}} \)
Light of energy \( U \) carries momentum \( U/c \). A non-reflecting surface absorbs the light completely, so the momentum delivered to it is just this value (a perfectly reflecting surface would receive twice as much).
\( \displaystyle U = (\text{flux}) \times (\text{area}) \times (\text{time}) \)
\( \displaystyle I\,A = 18\,\mathrm{W\,cm^{-2}} \times 20\,\mathrm{cm^2} = 360\,\mathrm{W}, \qquad t = 30\,\mathrm{min} = 1800\,\mathrm{s} \)
\( \displaystyle U = 360 \times 1800 = 6.48\times10^{5}\,\mathrm{J} \)
\( \displaystyle p = \frac{U}{c} = \frac{6.48\times10^{5}}{3\times10^{8}} = 2.16\times10^{-3}\,\mathrm{kg\,m\,s^{-1}} \)
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic was removed from the latest NCERT book (2026-27), and it also appears to be out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Nature of Electromagnetic Waves – …
