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Question

A lens having focal length f and aperture of diameter d forms an image of intensity I. Aperture of diameter d2 in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively.
  1. f and I4
  2. f and 3I4
  3. f and I2
  4. 3f4 and I4

A
3f4 and I4
B
f and 3I4
C
f and I4
D
f and I2
Solution
Verified by Toppr

By covering aperture, focal length does not change.
But intensity is reduced by 14 times, as aperture diameter d2 is covered.
I=II4=3I4
New focal length =f and intensity =3I4.

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