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  4. A man suffering from myopia can read a book placed at 10 cm distance. For reading the book at a distance of 60 cm with relaxed vision, focal length of the lens required will be

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Q. A man suffering from myopia can read a book placed at $10\, cm$ distance. For reading the book at a distance of $60 \,cm$ with relaxed vision, focal length of the lens required will be

Ray Optics and Optical Instruments

Solution:

For lens $u=$ wants to $see =-60 \,cm$
$v=$ can see $=-10 \,cm$
$\therefore \frac{1}{f}=\frac{1}{v}-\frac{1}{u} $
$\Rightarrow \frac{1}{f}=\frac{1}{-10}-\frac{1}{(-60)}$
$ \Rightarrow f=-12\,cm$