Question

A thin lens has a focal length f, and its aperture has a diameter d. It forms an image of intensity I. Now the central part of the aperture up to diameter (d/2) is blocked by an opaque paper. The focal length and image intensity would change to

(A) f/2, I/2 

(B) f, I/4 

(C) 3f/4, I/2 

(D) f, 3I/4

Answer 1

The correct option is (D) f, 3l/4.

Explanation:

Area of mirror,

Focal length will not change and intensity become 3/4 I.