How the eye works
Vision begins when light enters the eye. As it enters the eye, it is first bent, or refracted, by the cornea -- the clear window on the outer front surface of the eyeball. After the light passes through the cornea, it is bent again by the lens inside the eye. Tiny muscles in the eye change the shape of the lens and bring near and far objects into focus. This adjustment in the lens, is known as accommodation. In order to see clearly, objects need to be brought to a focus point precisely on the retina of the eye.
The cornea's curvature is ideally matched to it's length in the normal eye. In a nearsighted eye, images of distant objects (street signs) are brought to a focus point in front of the retina. This can be caused by a cornea which is overly curved and/or an eye that is too long. When the light reaches the retina, a blurred image is seen because the light rays spread apart after the focus point. This condition is known as myopia.
In a farsighted eye, the cornea is not curved enough and/or the eye is too short. This combination of factors causes the focus point of the eye to be located behind the retina. When light reaches the retina, a blurred image is seen since the light rays have not been brought to focus prior to reaching the retina. This condition is known as hyperopia.
In an eye with astigmatism, the cornea has a non-spherical shape (like a football) and does not bring light to focus at a single point. Instead, it focuses images over a range of points producing a blurred image. Both nearsighted and farsighted eyes can also have astigmatism.
As we age, the lens continues to grow and becomes thicker and less pliable. As a result, accommodative ability decreases and it becomes difficult to focus on near objects. This condition is known as presbyopia and usually begins to affect most people after the age of 40. Presbyopia, myopia, hyperopia, and astigmatism can all be treated with corrective lenses.
Once the light is focused on the retina, it stimulates cells which send millions of electrochemical impulses along the optic nerve to the brain. The brain interprets the impulses, enabling us to "see" the object. Because the light rays cross while going through the cornea, the retina reads the image upside down—but the brain readjusts so you stay properly oriented.
