Vision Disorders - Description



Light that strikes the eyeball first passes through the cornea. The cornea is the tough, transparent covering at the front of the eye. It is shaped like a dome, with the top of the dome facing outward. Light then passes through the lens. The lens is located just inside the eyeball, behind the cornea. The lens has a double-convex shape. That is, it bulges outward in the middle.

The cornea and lens bend light as light waves pass through them. This process is known as refraction. In a normal eye, the light waves are brought to focus on the retina. The retina is a thin membrane at the back of the eye. Light waves cause chemical changes in the retina. These chemical changes set off a signal that passes through the optic nerve to the brain. The brain reads that signal as a visual image.

Muscles in the front of the eye can change the shape of the lens. These muscles adjust the lens shape to see objects close at hand or far away. This process is known as accommodation.

Visual disorders develop when the cornea and/or the lens do not properly focus light waves. The light waves do not come to a focus (that is, they do not all reach the same point) on the retina. They may come to a focus in front of the retina or behind the retina.

When light waves come to a focus in front of the retina, the person has myopia, or nearsightedness. People who are nearsighted can see objects close at hand, but they cannot see objects far away clearly. When light waves come to a focus behind the retina, the person has hyperopia, or farsightedness. People who are farsighted (hyperopic) can see objects far away but cannot clearly see objects close at hand.

In healthy eyes the cornea has a smooth, regular shape. Some people have corneas with an ellipsoidal shape, like a football or a squashed baseball. Such corneas scatter light waves across the retina, causing objects to appear as blurred images. This condition is called astigmatism.

Accommodation:
The ability of the lens to change its shape in order to focus light waves from distant or near objects.
Astigmatism:
A condition in which light from a single point fails to focus on a single point of the retina. The condition causes the patient to see a blurred image.
Cornea:
The clear, dome-shaped outer covering of the front of the eye.
Diopter:
The unit of measure used for the refractive (light bending) power of a lens.
Hyperopia:
Farsightedness. A condition in which vision is better for distant objects than for close ones.
Lens:
In the eye, a transparent, elastic, curved structure that helps focus light on the retina.
Myopia:
Nearsightedness. A condition in which far away objects appear fuzzy, because light from a distance doesn't focus properly on the retina.
Optic nerve:
A bundle of nerve fibers that carries visual signals from the retina to the brain.
Radial keratotomy (RK):
A surgical procedure in which the shape of the cornea is changed in order to correct myopia.
Refraction:
The bending of light waves as they pass through a dense substance, such as water, glass, or plastic.
Retina:
A membrane at the back of the eye that is sensitive to light and that converts light waves into signals sent to the brain by way of the optic nerve.

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