A lens having focal length f and aperture of diameter d forms an image of intensity I. Aperture of diameter ${d \over 2}$ in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively

Question

$f\,$ and ${I \over 4}$

${{3f} \over 4}$ and ${I \over 2}$

$f$ and ${{3I} \over 4}$

${f \over 2}$ and ${I \over 2}$

Solution

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