Rays of light from Sun falls on a biconvex lens of focal length f and the circular image of Sun of radius r is formed on the focal plane of the lens. Then

Options

A. Area of image is `pir^(2)` and area is directly proportional of f
B. Area of image is `pir^(2)` and area is directly proportional of `f^(2)`
C. Intensity of image increases if f is increased
D. If lower half of the lens is covered with black paper area will become half

Answer

B

Explanation

From the figure in `triangle` ABC, `tanbeta=(AB)/(AC)`
`implies AB=AC `tanbeta`,`2r=ftanbeta`
`implies Area of image`=pir^(2)proptof^(2)`