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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 up to diameter d/2 is blocked by an opaque paper The image intensity will change toChoose the correct answer:a. f/2 and I/2b. f and I/4c. 3f /4 and I/2d. f and 3I/4

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 up to diameter d/2 is blocked by an opaque paper The image intensity will change toChoose the correct answer:a. f/2 and I/2b. f and I/4c. 3f /4 and I/2d. f and 3I/4

Grade:12th pass

2 Answers

Arun
25750 Points
6 years ago

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,

A = pi * d²/4

Remaining area after covering,

A' = pi *(d²/4  - d²/16) = 3 * pi* d²/16

I/I' = A/A' = 4/3

I' = 3 I/4

Regards

Arun (askIITians forum expert)

Rishi Sharma
askIITians Faculty 646 Points
3 years ago
Dear Student,
Please find below the solution to your problem.

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,
A = pi * d²/4
Remaining area after covering,
A' = pi *(d²/4 - d²/16) = 3 * pi* d²/16
I/I' = A/A' = 4/3
I' = 3 I/4

Thanks and Regards

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