Eyes and Vision

In essence, the eye functions in a very simple manner. Light waves passes through the cornea, through the lens, and finally converging and forming an image at the cornea. An electrical signal is then sent to the brain.

The cornea plays an important role in focusing the light entering the eyes. It is composed of 5 layers of tissue. The first and most outer layer, the epithelium, is composed of regenetive cells that provide protection for the eyes, while most of the remaining layers provide structural strength. These layers refract light, providing for the eye's primary focusing power. Consequently, changes or irregularities of the cornea can cause vision problems such as myopia (near sightedness), hyperopia (far sightedness), and astigmatism.

Myopia occurs when the curvature of the cornea is too strong for the length of the eye, causing the light waves to converge before reaching the retina. The resulting image on the retina will be blurred. Hyperopia is the opposite of myopia. It is the result of having a retina of insufficient curvature for the length of the eye, converging the waves at a point behind the retina. Astigmatism is the result of an irregular corneal shape, causing problems in focusing both close and far objects. Normal corneas have a round shape while a person with astigmatism may have a cornea that is shaped more like the shape of a football. Most people with myopia have some degree of astigmatism.

Now, after passing through the cornea, the light waves also have to pass through the pupil, or the black spot of the eyes. In the dark, the pupil is enlarged to allow more light to enter, and in a bright environment, the pupil is shrunk to reduce the amount of light entering. This is all controlled by the iris, the colored part of the eye.

After passing through the pupil, the light waves must pass through the lens. This lens further refracts the light, making fine adjustments to its shape in order to better focus the image on the retina. As the person ages, the lens will stiffen and lose some of its ability to change shape, hampering the ability to focus on both close and far objects. This condition is called presbyopia and is a part of the natural aging process.

Once the lens makes the final focusing adjustments, the light will pass through the vitreous humor, a clear gelatin-like substance taking the space between the lens and the retina, and is finally received by the retina. The retina has two parts: the macula in the center of the retina and the peripheral retina sourrounding the macula. The macula is a mere 5 percent of the retina and provies us with the ability to see details. When you read or focus on an object, the macula is being used. The peripheral retina, taking up 95% of the surface area of the retina provides peripheral vision. It is 100 times less sensitive than the macula, providing only the ability to distinguish shapes and colors.

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