Ray Optics and Optical Instruments - Result Question 35

35. 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:

[2010]

(a) f and I4

(b) 3f4 and I2

(c) f and 3I4

(d) f2 and I2

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Answer:

Correct Answer: 35. (c)

Solution:

  1. (c) 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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