If we are to see clearly, both the cornea (a) and the lens (b) (figure 1) have to refract the rays of light (thereby the term refraction) so that they focus on the retina, a layer of sensitive cells which furnish the eye fundus.

There the light is converted into electric impulses which are transmitted via the optic nerve to the brain.

If the rays of light are not correctly focused on the retina, the image will be hazy. We call this fault a refraction deviation.

Glasses, contact lenses, or refractive surgery corrects or reduces this refraction deviation by changing the refraction of the rays of light in such a way that they are focused on or closer to the retina.

Astigmatism

This arises because the cornea is not perfectly round. This causes an uneven projection of the image around the retina.

This hazy image can only be corrected by the use of cylinders (in glasses or contact lenses).