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  4. A far sighted person has his near point 50 cm, find the power of lens he should use to see at 25 cm, clearly.

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Q. A far sighted person has his near point 50 cm, find the power of lens he should use to see at 25 cm, clearly.

AIIMSAIIMS 2011Ray Optics and Optical Instruments

Solution:

Here $u = - 25 \,cm, v = -50 \,cm$
We have $\frac{1}{f}=\frac{1}{-u}+\frac{1}{v}$
i.e, $\frac{1}{f}=\frac{1}{25}-\frac{1}{50}$ or $f = 50\, cm $
Power of lens he should use,
$ P=\frac{100}{f}=\frac{100}{50}=+2D$