A plane electromagnetic wave in free space has an intensity of $800 W m^{- 2}$ . What is the radiation pressure exerted on a perfectly reflecting surface?

medium MCQ 2 marks Class 12 · Physics · Electromagnetic Waves

Answer

5.33 \times 1 0^{- 6} Pa

The correct answer is $5.33 \times 1 0^{- 6} Pa$ — Calculate $P = \frac{2 \times 800}{3.00 \times 1 0 ^{8}} = 5.33 \times 1 0^{- 6} Pa$ .

Step-by-step solution

For a perfectly reflecting surface the pressure is $P = \frac{2 I}{c}$ .
Insert the given intensity $I = 800 W m^{- 2}$ and $c = 3.00 \times 1 0^{8} m s^{- 1}$ .
Calculate $P = \frac{2 \times 800}{3.00 \times 1 0 ^{8}} = 5.33 \times 1 0^{- 6} Pa$ .

Common mistake: Using

I / c

instead of

2 I / c

for a reflector

Concept tested: Radiation pressure on reflecting surface

Published 11 September 2026 · Last updated 16 September 2026 · How this solution was prepared and checked