A biconvex lens of focal length f forms a circular image of radius r of sun is focal plane. Then, which option is correct?

Options

A. `pir^(2)propf`
B. `pir^(2)propf^(2)`
C. If lower half part is covered by black sheet, then area of the image is equal to `pir^(2)//2`
D. If f is double, intensity will increase

Answer

b.

Explanation

`r=f tan theta`
or `r prop f`
`:. pi r^(2)propf^(2)`