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Q. A man can see only between $75 \,cm$ and $200 \,cm$. The power of lens to correct the near point will be

Ray Optics and Optical Instruments

Solution:

For improving near point, convex lens is required and for this convex lens
$u=-25 \,cm , v=-75 \,cm$
$\therefore \frac{1}{f}=\frac{1}{-75}-\frac{1}{-25} $
$\Rightarrow f=\frac{75}{2} cm$
So power $P=\frac{100}{f}$
$=\frac{100}{75 / 2}=+\frac{8}{3} D$