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A lens having focal length f and aperture of diameter d forms an image of intensity I . Aperture of diameter d 2 in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively

Options

  1. Af and 1 4
  2. B3 f 4 and I 2
  3. Cf and 3 I 4
  4. Df 2 and I 2

Correct answer

C. f and 3 I 4

Step-by-step solution

Intensity, I A^2 I ₂ I ₁ = [ A ₂ ~A ₁ ]^2= r ^2- r ^2 4 r ^2 = 3 4 I ₂= 3 4 I ₁ and focal length remains unchanged.

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