Myopia or short-sightedness is an eye disease in which the person is unable to see the far objects. It is because the parallel rays from the far objects meet behind the retina after passing through the crystalline lens of our eyes. It may be due to two reasons:
i) Lengthening of the eyeball from front to back.
ii) The crystalline lens of our eyes has become too convex.
It can be corrected by using a concave lens whose power is defined in negative units called Dioptre.
1 Dioptre= 1/focal length.
Note that the focal length must be expressed in meters.
Hyperopia is a disease in which the person is unable to see the near objects. There are two reasons for this;
i) Shortening of the eyeball from back to front
ii) Lens may become too flat with age.
It can be corrected using convex lens whose power is defined in positive units by convention.
Thus a person having positive power has Hyperopia and wears convex lens.
Ex- A +3 dioptre lens converges the rays at 1/3 metres.