Question

A lens having focal length f and aperture of diameter D forms an image of intensity I. Aperture of diameter D/2 in central region of lens is covered by Black Paper. Focal length of lens and intensity of image now will be respectively-

1. f/2 and I/2
2. f and I/4
3. 3f/4 and I/2
4. f and 3I/4

Answer 1

is question complete

Answer 2

Focal length remains unchanged but intensity is directly proportional to square of area considering lens as a circle applying we get new intensity =3I/4