NEET 2010 Physics question
A lens having focal length and aperture of diameter forms an image of intensity . Aperture of diameter in central region of lens is covered by a black paper. Focal length of lens and intensity of image now will be respectively
Answer: (3)
Solution
The focal length depends only on the curvature of the surfaces and the refractive index of the lens material, not on the aperture, so is unchanged. The intensity of the image is proportional to the area of the lens that transmits light. Original area . The covered central part has
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