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A lens having focal length f and aperture of diameter d forms an image of intensity I.Aperture
of diameter d in central region of lens is covered by a black paper. Focal length of lens and 2
intensity of image now will be respectively :
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f and I 4 -
3f and I 4 2 -
f and 3I 4 -
f and I 2 2
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Correct Option: C
| By covering aperture, focal length does not change. But intensity is reduced by | ||
| 4 |
| times, as aperture diameter | is covered. | |
| 2 |
| ∴ I' = I - | = | |||
| 4 | 4 |
| ∴ New focal length = f and intensity = | . | |
| 4 |
