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

Options

A. (a)`pir^(2) prop f`
B. (b)`pir^(2) prop f^(2)`
C. (c)if lower half part is covered by black sheet , then area of the image is equal to `pir^(2)//2`
D. (d)if f is doubled , intensity will increase

Answer

B

Explanation

r = f tan `theta implies r prop f therefore pir^(2) prop f^(2)`