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  4. If focal length of human eye is 2 cm, then find the focal length of contact lens, such that a combined focus system of focal length 2.5 cm is obtained after using contact lens.

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Q. If focal length of human eye is $2\, cm$, then find the focal length of contact lens, such that a combined focus system of focal length $2.5\,cm$ is obtained after using contact lens.

AIIMSAIIMS 2018Ray Optics and Optical Instruments

Solution:

Pnet = Peye + Plens
$\frac{1}{f_{net}}=\frac{1}{f_{eye}}+\frac{1}{f_{lens}}$
$\frac{1}{2.5}=\frac{1}{2}+\frac{1}{f_{lens}}$ or $f_{lens}=-10$ cm