A human eye which cannot see the near objects and distant objects clearly is said to suffer from a defect known as Presbyopia. Eye suffering from Presbyopia cannot read and write comfortably.

This defect arises due to the aging of a person. The ciliary muscles are weakened and the flexibility of the crystalline lens of the human eye decreases with age of the person. As a result, human eye is unable to focus on close as well as distant objects.

This defect can be corrected by using a bi-focal lens. A bi-focal lens consists of a concave lens which forms the upper surface of the bi-focal lens and a convex lens which forms the lower surface of the bi-focal lens. The upper surface of bi-focal lens (i.e. the concave lens) enables the person to see distant objects clearly and the lower surface of bi-focal lens {i.e. convex lens) enables the person to see the near objects.