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Word Count: 345
The human eye does alot more than allowing you to see It is very complex and has many parts and features which can have defects However to understand defects you must first know how the eye works First light passes through the cornea which is the transparent part of the sclera or white of the eye which is composed of tough fiberous tissue Behind the sclera is a watery fluid called the aqueous humor This fluid fills a cresent-shaped space which with the cornea helps bend the light toward the center of the eye Under the aqueous humor is the iris which gives the eye color The color of the iris has no effect on how you see and is inherited through genes The iris contols how much light is allowed to enter your by opening up further when it is dark and closing up more to block out some light when it is bright Everything that passes through the pupil which looks like a black dot is what you see Next the light passes through the lens The lens focuses the light rays onto the retina forming an image in reverse and upside-down Finally light-sensitive cells in the retina transmit the image via the optic nerve to the brain by electrical signals Then the brain flips the image so it looks right-side-up to you You can find a diagram of the above on page 3 page 1 The most common visual defects are nearsightedness and farsightedness In nearsightedness also known as myopia the eye is longer than usual This is corrected by using a concave lens to spread the light rays just enough to increase the eyes focal length Hyperopia also known as farsightedness is caused by a shorter than usual eye A convex lens increases light bending and returns the point of focus to the retina
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