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Concepts Of Physics MCQ Edition [Volume 2]PhysicsPhotoelectric Effect and Wave–Particle Duality

A beam of white light strikes a plane surface normally. The surface absorbs 70 % of the light while reflecting the remainder. Determine the force exerted on the surface by the beam if it carries a power of 10 W .

Options

  1. A6.7 10⁻⁸ N
  2. B3.3 10⁻⁸ N
  3. C5.7 10⁻⁸ N
  4. D4.3 10⁻⁸ N

Correct answer

D. 4.3 10⁻⁸ N

Step-by-step solution

The force exerted by a light beam on a surface is given by the rate of change of momentum of the photons. For a beam of power P incident normally, the initial momentum incident per second is P c . The surface absorbs 70 % and reflects 30 % of the light. The force due to the absorbed light is F_ a = 0.7 P c . The force due to the reflected light is F_ r = 2 0.3 P c = 0.6 P c . The total force exerted on the surface is F = F_ a + F_ r = 1.3 P c . Substituting P = 10 W and c = 3 10⁸ m/s : F = 1.3 10 3 10⁸ = 13 3 10⁻⁸

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