Question

The image of sun is formed on focal plane of
lens which is a circular in shape of radius
$r$
and
area
$\pi {r}^{2}$


(a)
$\pi {r}^{2} \ {proportional} f$

(b)
${r}^{2} \: {proportional} \: f$

(c) lens is covered half so half image is formed
(d) on increasing focal length, brightness of the
image increases​

Answer 1

From the avove figure, we can see that

r=f tan α

Hence πr^2 ∝f^2