Structure of the Eye
Just like a camera, your eye has a lens, exposure meter and automatic focus. The major parts of the eye are the cornea, iris, lens and retina. The sclera is the outer layer of the eye that protects the retina and inner layers. The cornea is transparent and allows light to flow through it to the pupil. The pupil opens and closes, regulating the amount of light that passes to your retina. The ciliary muscles are located behind the lens and contract or expand, changing the curvature, or focus, of the lens. The retina contains photoreceptor cells that receive light and are made of rods and cones. Rods help with black and white vision and cones aid color vision.
The Lens and Retina
When you look at an object, the light reflected from the object passes through your cornea and lens. The object is inverted, or turned upside down, and displayed on the retina. Through complex chemical processes, that visual information is sent to the brain, telling you what the object is. A normal eye with perfect vision sees objects in the distance because the lens opens and flattens, or becomes concave. When a normal eye sees objects that are close, the lens relaxes and shrinks, becoming convex. The lens bends the light sharply, and allows you to see things that are near.
Nearsightedness and Farsightedness
Sometimes the eye does not see as well as it should. There are various types of eye disorders, with nearsightedness and farsightedness being the most common, and the most easily treated. Nearsightedness means that the person can see up close but not far away. This is because the structure of the person's eye is too long. Farsightedness is where a person sees clearly when looking into the distance, but cannot focus on objects up close.
There are several ways to correct your vision. The traditional way is with glasses, although many people wear contact lenses. Glasses offer corrected vision for both near- and farsighted people. The lenses on a pair of glasses bend the light so that you can see correctly. Contact lenses work under the same principles that glasses do. An optometrist or opthamologist can perform an eye examination and prescribe corrective lenses for you.
How Glasses Work
For nearsightedness, glasses correct the problem of the eyeball being too long to focus upon a far away image projected onto the retina. The glasses offer a concave lens that bends light rays outward, which normalizes the eyeball. In farsightedness, the eyeball is too short to focus upon objects that are near. Glasses use a convex lens that bends the light inward before it reaches the eye's lens, thereby correcting vision.