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  4. Calculate the focal length of a reading glass of a person if his distance of distinct vision is 75 cm.

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Q. Calculate the focal length of a reading glass of a person if his distance of distinct vision is $75\, cm$.

KCETKCET 2015Ray Optics and Optical Instruments

Solution:

Given,
$V=-75 \,cm $
$u=-25 \,cm$
We know that, lens formula, we have
$\frac{1}{f}=\frac{1}{v}-\frac{1}{u} $
$\frac{1}{f}=\frac{-1}{75}+\frac{1}{25} $
$\frac{1}{f}=\frac{1}{25}-\frac{1}{75}$
$\frac{1}{f}=\frac{3-1}{75}$
$\frac{1}{f}=\frac{2}{75}$
Focal length of a glass,
$f=\frac{75}{2}$
$=375\, cm$