JEE Main 2022 · 28 July · evening · Single correct · +4 / −1
Sun light falls normally on a surface of area 36 cm² and exerts an average force of 7.2 × 10⁻⁹ N within a time period of 20 minutes. Considering a case of complete absorption, the energy flux of incident light is
- (a) 25.92 × 10² W / cm²
- (b) 8.64 × 10⁻⁶ W / cm²
- (c) 6.0 W / cm²
- (d) 0.06 W / cm²
Answer and solution
Answer (d) 0.06 W / cm²
Solution
Physics sir · 4 steps
Fully absorbed light pushes with force IA/c, so I = Fc/A, and the 20 minutes do not matter.
- Force from light. Light carries momentum. When it is completely absorbed, the force on the surface is the power falling on it divided by c. The power is IA, so F = IA/c.
I = Fc/A
- Put in numbers. Fc = 7.2 × 10⁻⁹ × 3 × 10⁸ = 2.16 W. Divide by the area: I = 2.16/(36 cm²) = 0.06 W/cm², which is 600 W/m².
- Time is a distractor. Flux is energy per second per unit area. The force is already an average, so the 20 minutes do not enter the calculation.
- So. The energy flux is 0.06 W/cm², option (d).
The idea to keep. For complete absorption, radiation pressure is I/c. For perfect reflection it is 2I/c. Flux is a rate, so a given time interval does not change it.
Answer from the NTA final answer key. Solution by PrepWiser.