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The human eye works much like a movie camera. It receives light from objects that it focuses and projects onto a sensitive screen, and then converts the images into electrical impulses that are sent to the brain via the optic nerve.
The human eye works much like a movie camera. It receives light from objects that it focuses and projects onto a sensitive screen, and then converts the images into electrical impulses that are sent to the brain via the optic nerve. On the surface of your eye, the transparent cornea covers the colored iris and the nearly black pupil. The pupil controls light entering the eye. It dilates in low light conditions, thereby allowing more light to enter the eye. In bright conditions, it contracts to limit the amount of light let in.
The space between the cornea and lens is filled with a fluid called aqueous humor, which provides nourishment for the cornea and lens and keeps the eyeball round and firm. Just behind the iris and pupil is the crystalline lens, which converges light rays to focus on the retina. The shape of this elastic lens changes with the contractions and relaxations of muscles in the eye. The large chamber behind the lens is filled with a fluid known as vitreous humor. At the back of the eye is a thin membrane called the retina, which contains rods used primarily for nighttime vision and cones used for daylight vision. When light reaches the rod or cone cells, it causes a chemical change in light-sensitive pigments found within the cells. The chemical change results in an electrical impulse that is transmitted to nerve cells that branch into many fibers to form the optic nerve. The impulse is carried from the optic nerve to the visual centers of the brain where we perceive the image.
When the eye views distant objects, the crystalline lens becomes flat as it is pulled by the suspensory ligaments. When viewing closer objects, the ciliary muscle contracts, reducing the pull of the suspensory fibers and allowing the lens to bulge at the center. The rounder lens has a stronger focusing power, allowing objects to come into sharp focus on the retina. This increased focusing power necessary for near vision is known as accommodation. As we grow older, accommodation is gradually lost as the lens begins to harden. By age 40, a person may require eyeglasses for reading and other close-up work.