Question

A thin lens forms image of intensity 1. If the central
part of the lens of diameter half the diameter ol
lens is black polished, then what will be the
intensity of image formed by the remaining part?
​

Answer 1

Focal length will remains same. 

Intensity is directly proportional to the area of the aperture of the lens receiving  the light. 

Total area of the aperture is,

A = pi * d²/4

Remaining area after covering,

A' = pi *(d²/4  - d²/16) = 3 * pi* d²/16

I/I' = A/A' = 4/3

I' = 3 I/4