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A thin lens focal length f(1) and its ap...

A thin lens focal length `f_(1)` and its aperture has diameter `d`. It forms an image of intensity `I`. Now the central part of the aperture up to diameter`(d)/(2)` is blocked by an opaque paper. The focal length and image intensity will change to

A

`f//2,I//2`

B

`f,I//4`

C

`2f//4,I//2`

D

`f,3I//4`

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