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

A small, totally reflecting plane mirror is positioned horizontally to face a parallel beam of light, as shown in the figure. The mirror has a mass of 20 g . Assuming no absorption occurs in the lens and that 30 % of the light emitted by the source passes through the lens, determine the power of the source required to support the weight of the mirror. Take g = 10 m s ⁻² .

Options

  1. A2.0 10^8 W
  2. B3.0 10^7 W
  3. C6.0 10^7 W
  4. D1.0 10^8 W

Correct answer

D. 1.0 10^8 W

Step-by-step solution

Let P be the power of the source. The power of the light that passes through the lens and is incident on the mirror is P' = 0.30 P . Since the mirror is totally reflecting, the force exerted by the light beam on the mirror is given by the rate of change of momentum of the photons: F = 2P' c = 2(0.30 P) c = 0.60 P c For the light to support the weight of the mirror, this force must be equal to the gravitational force acting on the mirror: F = mg Substituting the given values ( m = 20 g = 0.02 kg , g = 10 m s ⁻² , an

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