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The radiation pressure exerted by a 450 W light source on a perfectly reflecting surface placed at 2 m away from it, is

Asked in JEE Main 3rd April 1st Shift 2025 · Force and radiation pressure

• (1) 1.5 × 10⁻⁸ Pascals
• (2) 3 × 10⁻⁸ Pascals
• (3) 0
• (4) 6 × 10⁻⁸ Pascals

Step-by-step solution

Why the other options are wrong

• (1) 1.5 × 10⁻⁸ Pa is a quarter of the answer — you put r = 4 m into P/(4π r²); the surface is 2 m away
• (2) 3 × 10⁻⁸ Pa is I/c — that is an absorbing surface; reflection doubles the pressure
• (3) comes from a perfect reflector does not escape the pressure — reversing the photon momentum doubles it, it does not cancel it

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