A lens having focal length f and aperture of diameter d forms an image of intensity I.
Apreture of diameter in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively:
f and
and
f and
and
Lensmaker's Formula -
- wherein
refractive index of medium of object
refractive index of lens
are radius of curvature of two surface
Focal length of lens depends on material and its radius of curvature. Hence focal length will remain same
Area covered is of total area hence of intensity will be blocked.
Intensity received =
focal length =f
& intensity =
Option 1)
f and
This is incorrect option
Option 2)
and
This is incorrect option
Option 3)
f and
This is correct option
Option 4)
and
This is incorrect option