The Ophthalmic Center - Refraction

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Refraction is the passage of light through a medium of varying density. The following section offers a simple overview of how light passing through the eye affects the quality of vision

The Normal Eye

Light rays are reflected off objects. When we look at an object the light rays enter the eye and fall on the back of the eye, or the retina. The retina then translates the light rays into electrical impulses that the brain interprets as visual images. A refractive error occurs when the rays of light that enter the eye do not fall correctly on the retina resulting in blurred vision. There are three types of refractive error; nearsightedness (myopia), farsightedness (hyperopia), and astigmatism. When light enters the eye it first passes through the cornea, the transparent covering of the eye. The cornea begins the process of refraction by bending the light rays that pass through it. The light rays pass through the pupil of the eye, which is a hole that expands or narrows depending on the amount of light entering it. Light then travels through the crystalline lens of the eye that narrows and refines the rays even more so, producing clearer images and focusing of images close up. This process is called accommodation and decreases with age. The result of the eye's decrease in accommodation is a condition called presbyopia and results in the need for reading glasses to see things close up. Light then passes through the transparent fluid in the inner eye, which also helps to focus the light rays even further. The light finally reaches the retina where it is sent to the brain.

Nearsightedness (Myopia)

If light rays fall in front of the retina because the curvature of the cornea is too steep, vision will be blurry when objects are seen at a distance. This condition is called nearsightedness (myopia) and is remedied by the use of eyeglasses or contact lenses that assist the cornea in bending light rays so that they fall correctly on the retina.

Farsightedness (Hyperopia)

If light rays fall behind the retina because the curvature of the cornea is too flat, vision will be blurry when objects are seen at a distance and close up. This condition is called farsightedness (hyperopia) and is remedied by the use of eyeglasses or contact lenses that assist the cornea in bending light rays so that they fall correctly on the retina.

Astigmatism

The shape of the eye is usually spherical much like that of a basketball. If the eye is shaped more like a football then the images do not fall correctly on the retina and appear distorted. This condition is called astigmatism and may be modified or corrected with the use of contact lenses or eyeglasses. Although it may sound disconcerting, astigmatism is quite common and usually is accompanied by some amount of hyperopia, or myopia.
We use a computerized device called an autorefractor to obtain a close estimate of a patient's refractive error. This device utilizes a sensitive scanning laser to read the focusing capacity of the eye. The information is printed out and the technician and doctor will use it in their further evaluation of the patient.

All three refractive errors, myopia, hyperopia and astigmatism are correctable with refractive surgery. Please refer to the LASIK section of our website for additional information.