Optics 6 Question 17
19. A biconvex lens of focal length $f$ forms a circular image of radius $r$ of sun in focal plane. Then which option is correct?
(a) $\pi r^{2} \propto f$
$(2006,3 M)$
(b) $\pi r^{2} \propto f^{2}$
(c) If lower half part is convered by black sheet, then area of the image is equal to $\pi r^{2} / 2$
(d) If $f$ is doubled, intensity will increase
Show Answer
Solution:
$$ \text { or } \quad r \propto f $$
$$ \therefore \quad \pi r^{2} \propto f^{2} $$
