Question

A person is able to see objects clearly only when they are lying at distances between $50cm$ and $300cm$ from his eye.

a. What kind of defect of vision is he suffering from?

b. What kind of lenses will be required to increase his range of vision to infinity?

Answer 1

a. The explanation for the defect of vision:

1. A normal eye can see from $25cm$to infinity. If a person can see in between $50cm$ and $300cm$ only, it means that he is suffering from both the defects of Myopia and Hypermetropia.
2. $25cm$ is the shortest distance and infinity is the longest distance for the normal eye.

b. The correction for increasing the range of vision of the person:

1. For increasing distance vision, a concave lens should be used.
2. For increasing the short vision, a convex lens should be used.
3. A person is suffering from both vision defects. So, a combination of the concave and convex lenses should be used. Thus, a bifocal lens should be used.
4. In a bi-focal lens, the upper portion is concave which corrects distant vision and the lower portion is convex which corrects near vision.