Your eye is made up of three different layers, the sclera, the choroid, and the retina (all of them shown above). The sclera maintains the shape of eye and contains the cornea where all light passes through. The choroid is the second layer of the eye. This layer is extremely important because it provides blood to all the structures of the eye (howstuffworks.com). Without blood nothing will be able to function. The choroid contains two parts to it, the iris and the ciliary body. The iris is the adjustable colored diaphragm around the pupil which is the black opening on the eye. The ciliary body controls the size of the lens by contracting and relaxing it. The innermost layer of the eye is the retina. The retina contains two vital parts to it, rods and cones. The cones are responsible for color vision and detail while the rods are responsible for vision in low light. Another important part to the retina is the chemical it contains called rhodopsin. This chemical converts light into electrical impulses which the brain interprets as vision (howstuffworks.com). Below is a picture of a rod and a cone.
Seeing color also deals with the cones in our eyes. There are color-responsive chemicals in the cones that are called cone pigments and are very similar to the chemicals in the rods. There are three kinds of color sensitive pigments: red-sensitive pigment, green-sensitive pigment, and blue-sensitive pigment. Each one of the cones cells in each of our eyes has one of these pigments so that it is sensitive to that color. The human eye is amazing in that it can sense almost any gradation of color when, red, green, and blue are mixed (howstuffworks.com).
Normal vision is something doctors came up with after studying many people looking and trying to read off letters on a chart that is twenty feet away from them. 20/20 vision means that when you stand twenty feet away from the chart you can see what a “normal” human being can see. However, if you have 20/40 vision it means that you are no longer “normal” because you have to be twenty feet away from something to read or see it when a “normal” person can be forty feet away from it to see or read it. 20/200 vision is absolutely horrible vision and is the legal cutoff for blindness in the United States. People can also have better vision than the norm. If you have 20/10 vision then you can see something at twenty feet away when most “normal” people have to be ten feet away to see it (howstuffworks.com).
Some problems with the eyes include nearsightedness, farsightedness, and even blindness. Nearsightedness is when you can see objects up close well but far away objects are tough to see. This is caused by an eye ball that’s too long, or a lens system that has too much power to focus (howstuffworks.com). This problem can be corrected with the use of a concave lens as seen below:
Farsightedness is just the opposite. This occurs when a person is able to see distance objects especially well and has trouble seeing close up objects. This is caused by an eye ball that is too short, or by a lens system that has too little focusing power (howstuffworks.com). This problem can be corrected with the use of a convex lens as seen below:
There are many things that can lead to blindness. Cataracts, glaucoma, or just trauma to the eye are only a few of the things that can lead to blindness. A cataract is cloudiness in the lens that blocks light from reaching the retina. This condition usually happens as we age. As it worsens, it can require surgery to remove the lens and replace it with an intraocular lens. Glaucoma is caused when the aqueous humor of the eye does not drain out correctly. This causes pressure to build up on the eye. When this happens it causes cells and nerve fibers in the back of the eye to break and die. This can be treated with surgery or medications only. Another way to go blind would be by receiving trauma to the eye itself (howstuffworks.com). Getting hit, being poked, or having certain chemicals getting into the eyes can prevent adequate vision.
Brain, Marshall. “How Stuff Works”. March 02, 2008
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