Question

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

• A. Area of image is $\pi r^2$ and area is directly proportional of $f$
• B. Area of image is $\pi r^2$ and area is directly proportional to $t^2$
• 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 1

The correct option is B Area of image is $\pi r^2$ and area is directly proportional to $t^2$

As shown in figure,

$r=f\tan \theta$

$r\propto f$

$\therefore Area=\pi r^2\propto f^2$