Eye Refraction Tutorial

The human eye works by allowing light to enter the eye through the clear lens, which then bends (or refracts) the incoming light and focuses it at the rear of the eyeball on the photosensitive retina. This visual information is then sent to the brain via the optic nerve. How well the eye refracts light determines its ability to focus light, which is directly responsible for the sharpness of your vision.

Anatomy of the Eye

• The eye is basically a jelly-filled globe with a clear lens and iris at the front, the retina on the back, and the optic nerve leading out of the back of the eyeball to the brain. Light enters at the front of the eye through the clear lens. The iris, which is able to expand and contract, controls the amount of light entering the eye. The incoming light then passes through the center of the eyeball, through a viscous jelly called the vitreous body, and is focused onto the rear of the eyeball, called the retina. Photosensitive cells on the retina send the visual information from the eye to the brain.

Eye Refraction

• Physical science shows us that when light crosses from one medium to another, it is refracted. So as light enters your eye, it crosses from air into the eye and is refracted. Light is bent most notably at the cornea, the outer curved surface of your eye. Medical writer Marilyn Haddrill says that both the length of the eyeball and the curvature of the cornea are the two main anatomic features that control the eye’s ability to focus incoming light. The lens and the eye’s internal jelly also help with focus.

Astigmatism

• Ideally, the cornea is a smooth symmetrical shape, and light entering from all angles is refracted the same amount. In reality, the cornea is sometimes asymmetrical. This irregular shape of the cornea, and even sometimes of the lens itself, causes light rays to be unfocused when they hit the retina, which results in blurry vision. This refractive error is known as astigmatism, and according to the American Optometric Association, it is a relatively common visual complaint. Small amounts of astigmatism usually cause no problems, but large amounts may lead to headaches and eyestrain.

Myopia and Hyperopia

• Nearsightedness and farsightedness are two conditions that result when the eye is unable to refract light so that it focuses sharply on the retina. In myopia (nearsightedness), the eyeball is too long, causing the light rays to focus before they reach the retina. In hyperopia, the eyeball is too short, causing the light rays to focus behind the retina. For both myopia and hyperopia, corrective lenses and laser surgery are available.