The cornea is the clear, dome-shaped window that forms the front wall of the eye. It acts as a lens to focus incoming light rays onto the retina, the light-sensitive tissue at the back of the eye.
In a normal eye the image focuses on the retina.In the normal eye, light rays are brought to a single sharp focus directly onto the retina, resulting in clear vision without glasses or contact lenses.
Any deviation from this normal focusing is called a "refractive error". Myopia, farsightedness, and astigmatism are the common refractive errors.
In the nearsighted eye the image focuses in front of the retina.In myopia, or nearsightedness, the eye is longer than normal. As a result, light rays come together at a point in front of the retina, and are thus out of focus on the retina.
Distant objects appear blurry, while near objects may be seen clearly.Myopia affects approximately one fourth of adults in the U.S.
In the astigmatic eye the rays of light do not come to a single point on the retina.In astigmatism, the curvature of the cornea (and therefore its focusing power) is not the same in the horizontal and vertical directions.
As a result, light rays entering the eye do not focus at a single point, and thus cause distorted vision. Many people with myopia have some degree of astigmatism.
In the farsighted eye the rays of light come together at a point behind the retina.In hyperopia, or farsightedness, the eye is shorter than normal. Light rays come together at a point behind the retina, and are thus out of focus on the retina.
Objects at close range may appear blurry whereas distant objects may be seen clearly. Many people with hyperopia may also have some degree of astigmatism.
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