A thin lens has focal length f and its aperture has diameter d.It forms an image of intensity I.Now the central part of the aperture

upto diameter d/2 is blocked by an opaque paper .The focal length and the image intensity will change to?

Focal length will remains same. 

Intensity is directly proportional to the area of the aperture of the lens receiving  the light. 

Total area of the aperture is,

Remaining area after covering,

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 the image brightness and intensity decreases ! this is because the no of light rays passing the lens are less

i think the focal lengh remains the same ! not sure though

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focal length will also half...

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