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

Explanation

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