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A man's shortest distance of distinct vi...

A man's shortest distance of distinct vision is `20 cm`. What wil be the type and power of the spectacle lens which he would he would require to enable him to read a book at a distance of `60 cm` ?

Text Solution

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Here, `v = -20 cm, f = ?, u = -60 cm`
As `(1)/(f)=(1)/(v)-(1)/(u) :. (1)/(f) = -(1)/(20)+(1)/(60) = (-3+ 1)/(60)=(-2)/(60) = -(1)/(30)`
`f = -30 cm`. Therefore, lens must be concave.
`P = (100)/(f) = (100)/(-30) = -3.3` dioptre.
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