A thin lens has a focal length f 1 and its aperture has a diameter d . If 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

Question

A thin lens has a focal length f 1 and its aperture has a diameter d . If 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, f 2 and I 2, f and I 4, 3 f 4 and I 2, f and 3 I 4

MARKS App · Accepted Answer

The focal length of the lens remains unchanged by changing its aperture. The intensity of the image is proportional to the uncovered area. I f I i = A f A i = π 4 d 2 - d 2 4 π 4 d 2 = 3 4 I f = 3 4 I i = 3 4 I

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