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

• A

  $\pi r^2 \propto f$
• Correct answer
  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

Solution

$r=f \tan \alpha$
Hence, $\pi r^2 \propto f^2$

Hence, the correct answer is option: 'B'.