A lens having focal length f and aperture of diameter d forms an image of intensity I. Aperture of diameter d/2 in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively
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a
f and I4
b
3f4 and I2
c
f and 3I4
d
f2 and I2
answer is C.
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Detailed Solution
focal length does not change, f ' = f⇒But I∝A, ∴I1I2=A1A2 ∴II2=πd2πd2-πd/22∴I2=3I4