Geometrical OpticsHard

Question

A biconvex lens of focal length f forms a circular image of sun of radius r in focal plane. Then

Options

A.πr2 ∝ f
B.πr2 ∝ f2 
C.if lower half part is covered by black sheet, then area of the image is equal to πr2/2
D.if f is doubled, intensity will increase

Solution

           
r = f tan α
Hence, πr2 ∝ f2

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