The eye has two structures that focus light on the retina: the cornea, which is the front of the eye, and the lens, which lies behind the iris and pupil. From the name, I would expect the lens to do most of the focusing work, but this is not the case. The cornea provides about two thirds of the refractive power and the lens about one third. Only the lens, however, can change shape to focus at different distances.
Both the cornea and the lens can become cloudy and block light. A cloudy or even opaque lens is called a cataract. The surgical procedure to treat cataracts is the most common surgery performed in the U.S., with a success rate of about 98 percent for millions of patients per year. In the past, patients had to wear thick glasses to replace the focusing power of the natural lens. Today cataracts are replaced with plastic lenses inside the eye. The newest generation of intraocular lenses even has some capability to change focal length, so that the patient can see at different distances without glasses. If you're near-sighted before cataract surgery, your doctor will choose replacement lenses that correct your vision. Some patients get monovision replacement lenses, so that one eye focuses correctly for distance vision and the other for reading vision. Your doctor may recommend that you try this option with contact lenses before surgery.
Cloudy cornea does not have a special name, as cloudy lenses do. There are no plastic corneas available yet; the treatment is a cornea transplant from a cadaver. This surgery is also very common, being performed about 40,000 times per year in the United States. Corneas are accepted much more readily than other transplants (heart, kidney, etc.), but still approximately 20 percent of corneal transplants are rejected.
Corneas are more exposed to the environment than lenses are, so there is a greater variety of problems that can cause cloudiness. Abrasion, infection, and inherited dystrophies are all possible. In the case of surface injury, current progress in laser surgery provides options to repair many eyes that would otherwise need corneal transplants.