Light with an energy flux of \( 22\,\mathrm{W\,cm^{-2}} \) falls normally on a non-reflecting surface of area \( 20\,\mathrm{cm}^2 \). The total amount of energy falling on the surface during a 30 minute time span will be:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → \( 7.92\times10^{5}\,\mathrm{J} \)
Energy flux is the energy falling per unit time per unit area, so the energy falling on the surface is flux × area × time. Because the light is incident normally, the whole area faces the beam.
\( t = 30\,\mathrm{min} = 30\times60\,\mathrm{s} = 1800\,\mathrm{s} \)
\( E = (\text{flux})\times A\times t \)
\( E = 22\,\mathrm{W\,cm^{-2}}\times20\,\mathrm{cm^{2}}\times1800\,\mathrm{s} = 440\,\mathrm{W}\times1800\,\mathrm{s} \)
\( E = 7.92\times10^{5}\,\mathrm{J} \)