Question

A biconvex lens of focal length $f$ forms a circular image of sun of radius $r$ in focal plane. Then,

• A. $\pi r^2\propto f$
• B. $\pi r^2\propto f^2$
• C. if lower half part is covered by black sheet, then area of the image is equal to $\pi r^2/2$
• D. if $f$ is doubled, intensity will increase

Answer 1

Answer: (B)

$r=f\tan \alpha$

Hence, $\pi r^2\propto f^2$