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A thin lens focal length f and its apert...

A thin lens focal length `f` 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

Text Solution

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Focal length of lens does not depends on the aperture of lens, It only depends on the
(i) Refractive index of material lens and surrounding medium and
(ii) Radius of curvature
For intensity `I prop (aperture diameter)^2`
`implies I_(blocked) prop (d/2)^2 and I_2 prop d^2` Hence, `I_(blocked)=I_0/4`
Final image intensity
`=I_(origi nal)-I_(blocked)=I_0-I_0/4=(3I_0)/4`
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