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Q80 (NDA-II/2020) Science & Technology › Basic Science (Physics, Chemistry, Biology) › Electricity and magnetism

Light of uniform intensity impinges perpendicularly on a totally reflecting surface. If the area of the surface is halved, the radiation force on it will become

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Your answer: —  ·  Correct: B
Explanation

Radiation pressure (P) is defined as the force exerted per unit area by electromagnetic radiation. For a totally reflecting surface at normal incidence, the radiation pressure is given by P = 2I/c, where I is the intensity and c is the speed of light. The radiation force (F) is the product of this pressure and the surface area (A), expressed as F = P × A. Since the light intensity is uniform and the surface remains totally reflecting, the pressure remains constant. Therefore, the radiation force is directly proportional to the surface area (F ∝ A). If the area of the surface is halved, the total force exerted by the radiation on that surface will also become half of its original value. This relationship is fundamental in calculating forces for applications like solar sails.

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