Vision and the Human Eye

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Shape. Colour. Depth. Brightness. Distance. Which single organ in the human body allows one to perceive these entities? The human eye. The eye is the most important organ in discovering the world around us. Although the eye measures only .5 centimeters in diameter, it can see as far away as a star and as tiny as a grain of sand. It can quickly adjust its focus between distant and near points, even while the head is moving. We use our eyes in everything we do- from reading and working to watching movies and playing games. Therefore, sight is a very precious, and fragile, sense. Though many do not ponder it, the ability to see and see clearly is quite a phenomenon. Here is an explanation of the structure of the human eye, how the eye sees, and some deficiencies that impair proper vision.

THE HUMAN EYE

When compared to the eye of various other species that can only differentiate between light and dark, the human eye is a fairly complex organ. Capable of distinguishing minute variations in shape, colour, and more, this light-sensitive organ is comprised of many different components. The entire eye, often referred to as the eyeball, is safely shielded inside the head. The spherical structure is approximately .5 centimeters in diameter with a pronounced bulge on the front surface. The outer part of the eyeball is consists of three tissue layers. The outermost layer, the sclera, acts as a protective covering. Also located near the sclera in the front of the eyeball are the iris, the pupil, and the cornea. The middle layer is known as the choroid, while the innermost layer is the light sensitive retina. For light to enter the eye and be directed towards the retina, it must pass through the cornea and then the lens. The lens is a flattened sphere compiled of numerous transparent layers of fibers. It is connected to the ciliary muscle. This muscle changes the lens' focal length by flattening the lens or making it more spherically shaped.

HOW WE SEE

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Though it is the organ associated with vision, the eye does not actually do the seeing. That is the task of the brain. In fact, what the eye does is see light rays reflected off of or by objects.

Primarily, light rays enter the eye via the convex cornea. When leaving the cornea, which provides the majority of the refracting power of the eye, the light rays proceed through the aqueous humour and the pupil to the lens. The lens bends the light rays closer together. Next, the ciliary muscle compresses and relaxes to modify the lens shape and, furthermore, change the focal length of the lens. On leaving the lens, the light travels through the vitreous humour, a liquid that maintains the shape of the eyeball and finally reaches the retina. With the help of the light sensitive rod and cone cells, an image is formed on the retina. The retinal image is inverted, so the optic nerve transports the electrical signals from the retina to the brain. There it is reinverted, creating the visual image of the object.

Focusing

If the human eye is capable of viewing a multitude of things, why can't it focus on nearby objects as well as distant ones at the same time? The refracting power of the lens is constantly being modified as the eye moves its focus between objects. Light rays from nearby objects spread out while those from distant objects run almost parallel. Therefore, the lens must greater accommodate for the light rays from nearby objects. As a result of this accommodation, the lens becomes rounder, thicker, and more powerful. When the eye must accommodate for the light rays from distant objects, the lens becomes flatter. It is for this reason that the eye cannot form a distinct image of a nearby object and a distant object at the same time.

Depth Perception

Depth perception is the capability to determine distance and/or thickness of an object. As the eyes are set slightly apart, each eye sees objects from a slightly different angle. Consequently, each eye transmits a slightly different message to the brain through the optic nerve. Each side of the brain receives electrical information to create visual images from each eyeball. Finally, the brain combines all of the electrical information, creating one visual image and providing depth perception. Normal depth perception requires that the eyes work together in a process called fusion or binocular vision. In this process, the eye muscles move the position of the eyeballs so that the light rays entering the eye fall at similar points on each retina. If the images' placements do not coincide with one another, they will be seen as double.

VISION DEFICIENCIES

Many people struggle with their vision from childhood through to old age. That is why vision deficiencies are the most common of all physical disorders. More than 100 million people in the United States wear eyeglasses or contact lenses for correctional purposes, as certain defects of the eye cannot be cured. However, defects such as shortsightedness (Myopia), farsightedness (Hyperopia), and astigmatism can all be corrected with the aid of various lenses. All of these focusing defects can tire and strain the eyes due to the constant adjusting in attempt to refocus images. Eyestrain and headaches are common symptoms of necessary vision correction.

Shortsightedness (Myopia)

Characterized by sharp near vision, but blurry distance vision, Myopia is the most common vision defect and seems to develop in childhood or adolescence. This defect of the eye occurs when the eyeball is too long from front to back and the eye muscles cannot make the lens thinner. As a result, light rays from distant objects meet before the retina. Once the light rays do reach the retina, they form a blurred image. In order to correct Myopia, concave lenses are used. These lenses, which are thinner in the middle than at the edges, bring the light rays together at the retina.

In the 170s, a surgical procedure called radial keratotomy was developed to correct myopia by reshaping the curvature in the cornea. More recently, a procedure entitled LASIK surgery (laser in situ kermatomeleusis) was contrived to perform the same task of lessening the effects of myopia. In this procedure, lasers are used to vapourize part of the cornea, flattening it. However, with LASIK eye surgery, there is a tendency that the myopia will partially return six to twelve months after the surgery. Though these procedures may eliminate the need for corrective lenses, their long-term effects are still unknown.

Farsightedness (Hyperopia)

Hyperopia occurs when the eyeball is too short from front to back. Unless proper accommodation takes place, the light rays from objects at short distances entering the eye meet behind the retina, causing a blurred image. In a normal eye, the lens remains almost flat for distance vision and becomes thicker for focusing nearby objects. However, with hyperopia, the eye muscles cannot make the eye lens thicker, clearly focusing the image. Therefore, a hyperoic eye receives sharp distant images, but blurred near ones. For correction of farsightedness, double convex lenses are used.

Astigmatism

Astigmatism, a defect with which one has the inability to focus light in different planes at one time, is usually the result of a misshapen cornea. Due to the abnormal shape, the light rays from an object do not come together at one specific point on the retina. Instead, some rays focus on the retina, some rays focus before the retina, and some rays focus after the retina. This defect produces blurred vision both nearby and at a distance. To correct this type of problematic vision, doctors prescribe cylindrical corrective lenses. These lenses have greater bending power in one axis than in others.

CONCLUSION

The power and significance of sight is something that is often overlooked until it is affected. However, through knowledge and understanding about vision, human eye, and certain vision deficiencies, one can truly appreciate what it is to see. Whether it is visiting a new city or watching a summer sunset, the ability to see allows us to observe the amazing and exciting world in which we live. No other organ but the eye empowers one to comprehend shape, colour, and brightness. Without sight, how would one easily recognize anything? Sight permits freedom to explore and seek. For that reason, sight is perhaps the most precious sense of all.

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