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

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Solution:

$$ \text { or } \quad r \propto f $$

$$ \therefore \quad \pi r^{2} \propto f^{2} $$



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