Q. A biconvex lens of focal length $f$forms a circular image of radius $r$of sun in focal plane. Then which option is correct?
A) $\pi r^{2} \propto f$
B) $\pi r² \propto f^{2}$
C) If lower half part is covered by black sheet, then area of the image is equal to $\pi r²\ /\ 2$
D) If $f$is doubled, intensity will increase
Explanation
(b) $r = ftan\theta$
Or $\ \ r \propto f$
$\therefore\ \ \ \pi r^{2} \propto f^{2}$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.