Question

The near point of a defective eye is `25 cm` from the eye. Calculate focal length of the lens required by her to read a book placed at `20 cm`.

Answer 1

Here, `u = -20 cm, v = -25 cm, f = ?`
From `(1)/(f) = (1)/(v) - (1)/(u) = (1)/(-25) + (1)/(20) = (-4 + 5)/(100) = (1)/(100) , f = 100`.
The lens must be convex, as focal length is positive.