Q. 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 a black paper. Focal length of lens and intensity of image now will be respectively
A) f/2 and I/2
B) f and I/4
C) 3f/4 and I/2
D) f and 3I/4
Explanation
[Solution from source] Covering part of a lens does not change its focal length, which depends only on curvature and refractive index. The light gathered is ∝ area: the covered central part has diameter d/2, i.e. 1/4 of the area, so intensity becomes I − I/4 = 3I/4. I/4 would be the share of the covered part, not what remains. Answer (4).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.