A thin lens has a focal length f and its aperture has a 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 would change to

Question

A thin lens has a focal length f and its aperture has a 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 would change to, f 2 , I 2, f , I 4, 3 f 4 , I 2, f , 3 I 4

MARKS App · Accepted Answer

The focal length remains f , as any small portion of the lens forms a complete image of the object and has the same focal length. While the intensity of the image depends on the area as, Intensity ∝ A r e a 2 I 2 I 1 = A 2 A 1 I 2 I = π d 2 4 - π d 2 2 4 π d 2 4 I 2 I = 3 4 I 2 = 3 4 I

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