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

`(f)/(2), (1)/(2)`

`f,(1)/(4)`

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

`f,(3I)/(4)`

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