Question

A certain person has minimum distance of distinct vision of 150cm. He wishes to read at a distance of 25cm. What focal length glass should he use? What is the nature of eye defect? [3 MARKS]

Answer 1

Each point: 1 Mark

1. Given:

u = -25cm

v = -150cm

2. We know, $\frac{1}{f}$= $\frac{1}{v}$- $\frac{1}{u}$ , By substituting $\frac{1}{f}$= $\frac{-1}{150}$- $\frac{-1}{25}$

$\frac{1}{f}$= $\frac{1}{25}$- $\frac{1}{150}$ , $\frac{1}{f}$= $\frac{150-25}{25\times 150}$

$\frac{f}{1}$= $\frac{25\times 150}{125}$

On solving Focal length $f=30cm$

3. Focal length $f$ being positive, lens used is convex lens.

The defect is hypermetropia