Question

A biconvex lens of focal length $f$ forms a circular image of the radius $r$ of the sun in the 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 $\frac{\pi r^{2}}{2}$
• D. If $f \, $ is doubled, intensity will increase

Answer 1

Answer: (B)

$r=ftan\theta $
Or $ \, \, r \propto f$
$\therefore \, \, \, \pi r^{2} \propto f^{2}$
small angled parallel rays meet at the focal plane.