Optics Question 578

Question: A thin lens focal length f1 and its aperture has diameter d. It forms an image of intensity I. Now the central part of the aperture upto diameter d2 is blocked by an opaque paper. The focal length and image intensity will change to

[CPMT 1989; MP PET 1997; KCET 1998]

Options:

A) f2 and I2

B) f and I4

C) 3f4 and I2

D) f and 3I4

Show Answer

Answer:

Correct Answer: D

Solution:

IA2I2I1=(A2A1)2=πr2πr24πr2=34

I2=34I1 and focal length remains unchanged.



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