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.