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` ?
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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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