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Question

A thin lens of focal length f and its aperture diameter d, forms a real image of intensity I. Now the central part of the aperture of diameter (d2) is blocked by an opaque paper. The focal length and image intensity would change to

A
f2,I2
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B
f,I4
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C
3f4,I2
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D
f,3I4
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Solution

The correct option is D f,3I4
On blocking the central part of the lens, focal length does not change.
Intensity of the image is directly proportional to the area of the lens.
Initial area= A1=π(d2)2=πd24
Final area= A2=π(d2)2π(d4)2=πd24πd216=3πd216
Hence
I2I1=A2A1=34
I2=34I1=34I

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