Question

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

(a) What kind of defects of vision he is suffering from?

(b) What kind of lenses will he required to increase his range of vision from 25 cm to infinity ? Explain briefly.

Answer 1

1. For a normal eye, the far point is considered at infinity and the near point is taken at a distance of $25cm$ from the retina.
2. For the person mentioned here, the far point of his eye is $300cm$ and not infinite. So the person has shortsightedness.
3. Also, the near point of his eye is $50cm$ that greater than $25cm$ (the normal range). So the person has longsightedness.

(a) Therefore, the person is suffering from both shortsightedness (myopia) and longsightedness (hypermetropia).

(b) To increase his vision range from $25cm$ to infinity, the person needs both a concave and convex lens. It is also called a bi-focal lens. The concave lens corrects shortsightedness and the convex lens corrects longsightedness.