What focal length should the reading spectacles have for a person for whom the least distance of distinct vision is 50 cm ?

Explanation

The distance of normal vision is 25 cm. So if a book is at `u=-25cm.` Its image should be formed at `upsilon = -50cm`. Therefore, the desired focal length is given by
`(1)/(f)=(1)/(v)-(1)/(u) or (1)/(f)=(1)/(-25)=(1)/(50)`
`or f=+50cm` (convex lens).