Question

A certain person has minimum distance of distinct vision of 150 cm. He wishes to read at a distance of 25 cm. What focal length glass should he use? What is the nature of eye defect?

Answer 1

Minimum distance of distinct vision for normal eye is 25 cm
Given:
Minimum distance of distinct vision in this case = 150 cm

$u=-25cm$
$v=-150cm$

$\frac{1}{f}=\frac{1}{v}-\frac{1}{u}$
$\frac{1}{f}=\frac{-1}{150}-\frac{-1}{25}$
$\frac{1}{f}=\frac{1}{25}-\frac{1}{150}$
$f=+30cm$

Focal length $f$ being positive, lens used is convex lens. The defect is hypermetropia