A lens haivng focal length and aperture ...

A lens haivng focal length 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.

f and `1/4`

`(3f)/4` and `I/2`

f and `(3I)/4`

`f/2` and `I/2`

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The correct Answer is:

By covering aperture, focal length remains same. But intensity is reduced by `I/4`

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