A biconvex lens of focal length f forms a circular image of the radius r of the sun in the focal plane. Then which option is correct?

Question

A biconvex lens of focal length f forms a circular image of the radius r of the sun in the focal plane. Then which option is correct?, π r 2 ∝ f, π r ² ∝ f 2, If lower half part is covered by black sheet, then area of the image is equal to π r 2 2, If f is doubled, intensity will increase

MARKS App · Accepted Answer

r = f tan θ Or r ∝ f ∴ π r 2 ∝ f 2 small angled parallel rays meet at the focal plane.

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