A lens having focal length f and aperture of diameter d forms an… | NEET 2010 (AIPMT, Prelims)

Q103NEET 2010 · AIPMT · PRELIMSMedium

Spherical Surfaces & Thin Lenses

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

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Answer: (c)

Covering part of a lens does not change its curvature, so focal length stays $f$ .
Light through the lens $\propto$ open area. Covered area $= \pi(d/4)^2 = \frac{1}{4}\pi(d/2)^2$ , i.e. $\frac{1}{4}$ of the total.
Open area is $\frac{3}{4}$ of the original, so intensity becomes $\frac{3I}{4}$ .
So the answer is (c) $f$ and $\frac{3I}{4}$ .

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Spherical Surfaces & Thin Lenses question from NEET 2010 (AIPMT, Prelims)

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