When a person is nearsighted, or myopic, the light ray's focal point is somewhere before it reaches the retina.  Contacts can correct this problem by diverging the light back toward the retina.

When a person has hyperopia, or farsightedness, the light ray's focal point is somewhere after it reaches the retina.  Contact lenses can correct this problem by converging the light back toward the retina.

Sometimes eye abnormalities can occur that can cause vision impairments, including blindness, that contacts cannot correct.  One of the abnormalities is when the lens becomes cloudy.

Another abnormality that can affect vision is a change in the shape of the eyeball.  The eye can no longer focus light correctly and send the visual information through the optic nerve.

Sometimes a person's retina can start to deteriorate.  When this happens the eye is no longer able to get a clear picture.

The optic nerve can also become damaged.  When this happens no message is sent to the brain which results in blindness.  Sometimes blindness is inherited and other times it can be caused by disease or an accident.  At the present time, contacts cannot cure blindness.