The cornea is the clear front window of the eye. It vaults over the iris (the colored part) and pupil (the black center). The cornea is part of the tough wall of the eye, and because it is clear, it also focuses light. Light from the cornea is sent through the pupil into the lens, and then on to the retina. The shape and curvature of the cornea helps determine if the eye is nearsighted, farsighted or has some astigmatism. The cornea actually focuses over 50% of the light that enters the eye!
Because it is exposed to the air, like the windshield of an automobile, or outside of the TV monitor, the cornea is subject to considerable abuse from the outside world. Particles of dust and foreign materials inevitably find their way onto the eyes, irritating them and stimulating the production of tears to wash the foreign materials away. The cornea is often involved in dry eye conditions. Sometimes, due to long term irritation, other eye tissues can grow over or into the cornea, causing some blur and discomfort. The surface of the cornea needs to be wet, smooth and regular in order to obtain the best vision. Dry eye, eyelid disease and hereditary corneal diseases can cause visual blur that cannot be corrected with glasses or cataract surgery.
Although it might seem to be a very thin membrane, the human cornea actually has several distinct layers, some of which are quite strong.
The cornea is so well developed that only the most expensive human-made lenses can match its precision. However, the smoothness and shape of the cornea (inside and out) is also vitally important. If the surface smoothness or the clarity (shape or thickness) of the cornea are disturbed, then the vision becomes distorted. These sorts of things can happen because of corneal aging, dry eye issues, injury, and inherited conditions. Because the cornea is a powerful light focusing tissue, refractive eye surgery techniques (LASIK or PRK) can be used to change the shape of the cornea so as to reduce the need for corrective lenses and otherwise improve the “relaxed” focusing state of the eye.