A perfectly reflecting metallic plate is illuminated perpendicularly by an electromagnetic wave of intensity $I = 250 W m^{- 2}$ . What is the radiation pressure $P$ on the plate?

medium MCQ 3 marks Class 12 · Physics · Electromagnetic Waves

$1.67 \times 1 0^{- 6} Pa$
$5.0 \times 1 0^{- 7} Pa$
$2.5 \times 1 0^{- 4} Pa$
$1.25 \times 1 0^{- 3} Pa$

Answer

1.67 \times 1 0^{- 6} Pa

The correct answer is $1.67 \times 1 0^{- 6} Pa$ — Evaluate: $\frac{500}{3.00 \times 1 0 ^{8}} = 1.6667 \times 1 0^{- 6} Pa$ , which we write as $1.67 \times 1 0^{- 6} Pa$ .

Step-by-step solution

For a perfectly reflecting surface the radiation pressure is $P = \frac{2 I}{c}$ because the momentum change is twice that for absorption.
Substitute $I = 250 W m^{- 2}$ and $c = 3.00 \times 1 0^{8} m s^{- 1}$ : $P = \frac{2 \times 250}{3.00 \times 1 0 ^{8}}$ .
Evaluate: $\frac{500}{3.00 \times 1 0 ^{8}} = 1.6667 \times 1 0^{- 6} Pa$ , which we write as $1.67 \times 1 0^{- 6} Pa$ .

Common mistake: Using

P = I / c

instead of

2 I / c

for a reflecting surface

Concept tested: radiation pressure on reflecting surface

Published 20 September 2026 · How this solution was prepared and checked

← All Electromagnetic Waves questions

A perfectly reflecting metallic plate is illuminated perpendicularly by an electromagnetic wave of intensity I=250 W m−2. What is the radiation pressure P on the plate?

Step-by-step solution

A perfectly reflecting metallic plate is illuminated perpendicularly by an electromagnetic wave of intensity $I = 250 W m^{- 2}$ . What is the radiation pressure $P$ on the plate?