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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 I 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

Bycovering aperture, focal length does not change. But intensity is reduced by 1 4 times as aperture diameter d 2 is covered. array l I^ =I- I 4 = 3 I 4 New focal length = f and intensity = 3 I 4 . array

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