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Questions  

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

a
f and I4
b
3f4 and I2
c
f and 3I4
d
f2 and I2

detailed solution

Correct option is C

focal length does not change, f ' = f⇒But I∝A, ∴I1I2=A1A2 ∴II2=πd2πd2-πd/22∴I2=3I4

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