A lens having focal length f and apertur...

A lens having focal length f and aperture of diameter d forms an image of intenjsity I. Aperture of diameter `(d)/2`in central region of lens in covered by a black paper. Focal length of lens and intensity of image now will be respectively

`fandl/4`

`(3f)/4andl/2`

`fand(3l)/4`

`f/2and(l)/2`

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