The eye is one of the most complex organs in the body. Seventy percent of the body’s sensor receptors are located in the eyes. Vision is the process by which the eye takes in information and passes it to the brain to be perceived or interpreted.
How Does the Eye See?
Vision occurs when the brain interprets light that is reflected from an object. The light enters the cornea through the pupil. The pupil is located in the middle of the iris, the colored membranous disk that is composed of two types of muscular fibers. When exposed to light, the circular fibers contract and the radial fibers relax. When there is less light, the radial fibers contract and the circular relax.
The cornea causes the light rays to refract. This causes the image to partially focus. The light rays continue through the lens, which focuses the rays even more by changing shape. This change is actually carried out by the ciliary muscle. With faraway objects, the ring of ciliary muscle relaxes causing ligaments to pull on the lens. This movement causes the lens to become thinner and flatter. With nearby objects the ciliary muscle contracts. The ligaments stop pulling and the lens returns to its natural shape.
The light rays cross the inside of the eye and reach the retina, or inner lining of the eye that converts light into nerve impulses. The retina contains two types of cells. Rods handle vision in low light. They see only in black and white. Cones handle color vision and detail. People who are colorblind have difficulty distinguishing colors. This affliction is caused by the absence of the types of cone cells that are sensitive to blue, green, or yellow.
The retina produces an inverted image of the object that is sent to the brain by the optic nerve. The brain processes the information and constructs an image of the object. Vision is the term for the process of seeing this object.
Common Vision Problems and How to Correct Them
The two most common vision problems involve seeing objects that are out of focus. Both can be corrected with lenses, either in the form of glasses or contacts.
Hyperopia, also known as farsightedness, makes it difficult to see objects that are close to the eye. This vision problem occurs when the image is being focused behind the retina because the eyeball is too short. The light rays are not bent enough and reach the retina before they have been focused. Convex lenses placed in glasses can correct it. They allow the light to strike the retina properly by making the light rays converge.
Myopia, also known as nearsightedness, is a vision problem that occurs when the ocular sphere of the eye is to long causing the image of the object to form in front of the retina because the light rays are bent too much. This causes difficulty seeing distant objects, and may be corrected by glasses with concave lenses that make the light rays diverge.