The distance of distinct vision of a person is `40 cm`. Calcuate the power of the lens he should use to read newspaper at a distance of `25 cm`.

Explanation

Here, `v = -40 cm, u = -25 cm, P = ?`
From `(1)/(f) = (1)/(v) - (1)/(u) = (1)/(-40) - (1)/(-25) = (-5 + 8)/(200) = (3)/(200) , f = (200)/(3) cm`
`P = (100)/(f) = (100)/(200//3) = (3)/(2) D , P = 1.5 D`
As `P` is positive, the lens must be convex.