A lens having focal length f and aperture of diameter d forms an image of intensity I. Aperture of diameter d/2 in central region of lens is covered by black paper. Focal length of lens and intensity of image now will be respectively:
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Question

A lens having focal length f and aperture of diameter d forms an image of intensity I. Aperture of diameter d/2 in central region of lens is covered by black paper. Focal length of lens and intensity of image now will be respectively:

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G1 Physics · Accepted Answer

Step 1: Focal length depends on refractive indices and surface curvatures; obscuring aperture does not change f. Step 2: Total aperture area A = π(d/2)². Covered central disk diameter d/2 has area A_c = π(d/4)² = A/4. Step 3: Open area = A − A/4 = 3A/4. For uniform illumination the image intensity is 3I/4. Final answer: (C) f and 3I/4.