Question

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. $\pi r^2\propto f^2$
  
• 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

The correct option is B $\pi r^2\propto f^2$

We have $r=ftan\alpha$
Thus we get $r\propto f$ and $r^2\propto f^2$
$⟹\pi r^2\propto f^2$