The Human Eye
The eye is an intricate and sophisticated structure that enables humans to perceive the world; without which our world, literally, would be dark.
The eye has many complex sections that work together to create the images that we see everyday. Generally the eye is approximately one inch wide, one inch deep and 0.9 inches tall. The sclera is the outermost layer of the eye and its function is to maintain the shape of the eye. Six muscles are attached to the sclera that work together to control the movements of the eye: the medial rectus, which moves the eye towards the nose, the lateral rectus which moves the eye away from the nose, the superior rectus, which raises the eye, the inferior rectus, which lowers the eye, and the superior and inferior oblique, both of which rotate the eye (Bianco). Covering the sclera is a mucus membrane, called conjunctive, which keeps the eye moist.
The cornea is the clear, frontal section of the sclera, which allows light to pass into the eye. The second layer of the eye is the choroid, and supplies blood to the eye. The choroid is composed of two sections, the ciliary body and the iris. The ciliary body is a muscular section attached to the lens, it controls the focusing of the lens by contracting and relaxing. It also produces a liquid solution called aqueous humor, which is located in the front of the eye. The aqueous humor is one of two liquid solutions found in the eye, the other is vitreous humor, which is a gel like substance located in the back section of the eye. The iris is the colored part of the eye and contains two muscles, the dilator and the sphincter. When the dilator contracts, it makes the iris smaller, while the sphincter relaxes to make the iris larger. This controls the amount of light that is let into the eye through the pupil. The picture to the left is an example of the dilator at work, the iris is large, making the pupil small. While on the right is an example of the sphincter, the iris is small, making the pupil larger (Your).
The innermost layer of the eye is the retina, the retina is the light sensing area of the eye. Within the retina there are cells called cones and rods, which detect color and shapes respectively. There are approximately one hundred and fifty million cones and rods in the retina (Your). Both of these are important to our visual processing and will be discussed in more detail later on. The macula is also found in the retina, it is in the center of the retina, the macula contains the fovea centralis, which only houses cones. There is also the chemical rhodopsin, or “visual purple” within the retina, which converts light into electrical impulses that are sent to the brain for interpretation (Bianco). The electrical impulses are carried to the brain by the optic nerve, this is the collection of retinal nerve fibers that collect at the back of the eye. Passing through the optic disk , which is where the optic nerve and blood vessels exit the retina on their way to the brain.. When all of these sections are put together, the eye is formed and fully operational.
Like the eye itself, the process by which humans are able to perceive light and color is also complex. When we perceive light, it must first pass through the cornea, then the aqueous humor, lens and vitreous humor before reaching the retina (Bianco). The light is refracted slightly by the aqueous humor, then a little bit more by the lens and vitreous humor. The lens is responsible for forming the shape of the image, it is able to adjust quickly to different objects by the process of accommodation. The ciliary muscle relaxes and contracts, changing the shape of the lens to adjust to the objects size and distance (Polarization). Once the light reaches the retina it comes in contact with rods and cones, this contact causes chemical reactions to begin in the eye. Activated rhodopsin is created, as mentioned before this generates electrical impulses to the optic nerve. When light enters the rods, it contacts rhodopsin and causes it to decompose into all-trans retinal, which is unstable and decomposes further to metarhodospin II. This is the chemical that actually creates the electrical impulses that are transmitted to the brain (Bianco). When these messages are perceived in the brain, they appear upside down, it is then the brain’s job to flip the images right-side up, this process occurs in the visual cortex (Your).
While the process of perceiving light mainly occurs in the rods, the perception of color takes place largely in the cones. The actual process of perceiving color is similar to that of light, the main difference is that the chemical photopsin replaces scotopsin (see chart above). Also, there are three color-sensitive pigments within the cones, these pigments sense red, green and blue. Each cone has one of these pigments and by various blends of these pigments, we are able to perceive almost any color (Bianco). Both of these processes occur at the same time, allowing humans to perceive the light, shapes and colors of the world.
When all is functioning properly, humans are able to “see” the world around them clearly and in focus. Visual acuity is tested by the Snellen eye chart, which is a chart of letters in rows, decreasing in size from the top row.
http://www.contrastsensitivity.net/images/echart.jpg
Normal vision is defined as 20/20, which means that at twenty feet away from the chart you can see the same amount of accurate letters as most people see from twenty feet. 20/40 means that you have less than normal visual acuity and can only see what most people can see at forty feet away. It is possible to have better than normal vision, 20/10 for example. This means that at twenty feet you can see what most people can only see at ten feet away (Bianco). Depth perception is another part of normal vision, this enables humans to decipher how far away an object is. There are three main processes by which humans determine an object’s depth/distance. The first is the amount of space the object takes on the retina. If the object takes a large place on the retina, the brain will determine that it is closer, while an object that is farther away would take up less space on the retina. Secondly, when we move our head side to side, objects that are close to us move rapidly across our retina, and appear as a blear moving past us, while those that are farther away move across our retina at a slower pace and appear more clearly (Bianco). Here’s an example, imagine that you are looking out the car window, the trees that are right at the edge of the road appear in a blur of green and brown, while we can clearly see those that are farther away. Thirdly there is stereo vision, each eye receives a different image since they are about an inch apart. The brain interprets the differences in these images to determine the location of the object (Bianco).
Since the process of vision is complex, there can be many errors and problems that occur. One of which is color blindness, this is the inability to differentiate between colors. This affliction is more common in men, since it is an inherited effect and the capacity of color vision is in the X-chromosomes and women have two. The most common type of color blindness is the inability to differentiate between red and green. In this case either the red or green cones are not present or they do not function properly (Bianco).
http://www.colormatters.com/kids/images/cones_color.jpg
Another failure in vision is night blindness, which is caused by vitamin A deficiency. Vitamin A is necessary to form retinal, a part of the rhodopsin molecule that that senses light. A lack of these light sensing molecules causes an inability to see at night, since there is not enough light (Bianco). This is less common, but its does still occur. Astigmatism is more common, especially as people age, and is a curvature of the cornea that distorts vision. This can be corrected by creating a contact lens that corrects the curvature of the cornea. Troubles with refraction also cause visual problems such as nearsightedness and farsightedness. Nearsightedness, or myopia, is when a person can see things clearly up close, but not far away. This is caused by an eyeball that is, in essence, too long.
(Your)
The light rays don not quite reach the retina, and are focused instead, on the front of the retina. This can be corrected with concave-lenses, which will cause the light to diverge slightly before reaching the eye (Bianco).
Farsightedness, or hyperopia, is when a person can see things at a distance, but not close up. This, is the opposite of nearsightedness and is caused by an eyeball that is too short.
(Your)
The shortness of the eyeball causes the light rays to go pass the retina. This can be corrected with convex lenses (Bianco).
While these are minor problems that can be corrected for the most part, there are also many causes of blindness.
Blindness is technically defined as vision that is less than 20/200 and can have many different causes. One such cause is cataracts, which are cloudy substances that form on the lens, preventing light from reaching the retina. These are more common with age and can be corrected with surgery.
http://www.ehponline.org/docs/2005/113-3/eye.jpg
Glaucoma is another cause a blindness, this occurs when the aqueous humor does not drain correctly, causing pressure to build up in the eye killing cells and nerve fibers in the back of the eye. This can also be corrected with surgery or medicine (Bianco). Diabetic retinopathy also causes blindness in people with diabetes. It occurs when there is a blockage, leakage and/or scaring of blood vessels in the eye. However, it can be cured with laser surgery. There is also macular degeneration, the macular is responsible for fine-detailed vision, it begins to deteriorate, and eventually all central vision is lost. Sometimes this can he helped with laser surgery, but not always (Bianco). Trachoma is an infection that is caused by Chlamydia trachomais and eventually leads to blindness, however it can be cured with antibiotics. Another form of blindness is caused by retinitis pigmentosa, an inherited disease that causes blindness by degenerating the retina and excess pigment that allows humans to perceive light. It is a degenerate process, beginning with night blindness, then tunnel vision then complete blindness. There is, unfortunately, no known treatment for this disease. Trauma can also cause blindness, especially if it involves chemical injuries that damage the eye (Bianco). While these are some of the most prevalent causes of blindness, there are many others including strokes and neurological diseases. While these cases of blindness exists, most people are fortunate and have accurate vision, or vision that can be corrected with glasses or contact lenses.
The eye is a fascinating mechanism that is critical to man’s survival. Man’s existence is centered on the proper functioning of the eye, and today we are lucky to have the technology to correct many dysfunctions of the eye. The eye’s processes of vision, and the physics that accompanies it is an intriguing and necessary part of life.
Bibliography
Bianco, Carl. "How Vision Works." How Stuff Works. 26 Feb 2008 http://health.howstuffworks.com/eye.htm.
"Polarization; and The Huamn Eye." PY106-Elementary Physics II. Boston University. 29
Feb2008 http://physics.bu.edu/~duffy/PY106/Eye.html.
"Your Sense of Sight." Think Quest Junior. 29 Feb 2008
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