Refraction is the basic function of our eyes. Speaking simply it means bending of light rays to get focused at a point, we can not only perceive the light rays from objects situated far-off but also those objects which are close to our eyes.

To help in refraction our eyes have :

(1) The Cornea - A transparent layer set like a watch glass in front of our eyes.

(2) The lens - A biconvex lens which can change its curvature as per the need to focus the light rays.

Any defect in the curvature of lens, cornea or the size of eyeball leads to refractive errors. They include :

(a) Short sightedness or Myopia

(b) Long sightedness or Hypermetropia

(c) Astigmatism

(d) Presbyopia (it is basically an age change but can be included here as it too leads to defective vision).