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Contact lenses are curved thin discs generally about the size of a fingertip. They are transparent and may be colored. Worn directly on the eye, contacts are held in place by tears between the contact lens and the cornea. Direct contact with the eye eliminates the need for frames to hold the lenses in place, providing unobstructed peripheral vision as compared to spectacles. Contact lenses are medical devices that can be legally purchased only with a prescription.
The eye works like a camera with two lenses, absorbing and processing light reflected from your surroundings.
Light first passes through the cornea, a clear membrane that covers the front surface of the eye. Like a camera lens, it refracts light, helping to focus the light.
The light then passes through the pupil, the dark circular opening in the middle of the iris or colored part of the eye. Like the aperture in a camera, the pupil controls the amount of light entering the eye by becoming larger or smaller.
The light then passes through the eye's natural crystalline lens. This is a clear tissue located behind the pupil. The natural crystalline lens "fine-tunes" the image before it is focused on the retina at the back of the eye.
The retina works like the film in a camera. The retinal nerves absorb light and convert it into electrical signals. These signals are subsequently sent through the optic nerve to the brain where they are interpreted as visual images.
If both lenses are working properly, the light is focused precisely on the surface of the retina. If the two lenses are not working properly, clear vision may be achieved by refocusing light rays with the use of corrective lenses.
Different types of contact lenses are required to correct various conditions.
Myopia occurs when light rays are focused in front of the retina. Close objects can be seen clearly but objects at a distance are out of focus. Lenses that are thinner in the center than on the edges (concave) increase the focal length.
Hyperopia is a condition in which light rays are focused behind the retina. Distant objects can be seen clearly but close objects are out of focus. Lenses that are thicker in the center than on the edges (convex) decrease the focal length.
Astigmatism is a condition in which light is focused on two separate points in the eye. The distorted image is caused by an irregularly shaped cornea. Lenses are custom shaped to compensate for the irregular shape of the cornea.
Normally eye muscles are able to reshape the eye to bring objects at different distance into focus. With aging, the eye loses some of its elasticity. Bifocal or multifocal lenses have two or more different curves or shapes combined into one lens. Usually the bottom of the lens is for reading or viewing close objects, while the top of the lens is for seeing farther into the distance.
Contact lenses can be categorized according to what they are made of, and how often they are replaced. There are two main categories. Soft, or 'gel' contacts are easy to adapt to, comfortable and need to be replaced often. Rigid gas permeable contact lenses are durable, resistant to deposit buildup and provide some people with sharper vision.
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