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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) " and " (I)/(2)`

B

f and `(I)/(4)`

C

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

D

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

Text Solution

Verified by Experts

The correct Answer is:
D
Intensity (I) `prop ("area")^(2) `
`rArr (I_(1))/(I_(2)) = (pi r^(2))/(pi r^(2) - pi ((r )/(2))^(2) ) = (4r^(2))/(3r^(2)) rArr I_(2) = (3)/(4) I`
Focal length remains unchanged.
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