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The human eye works by sending light through a series of specialized parts to the optic nerve directly to the brain. Light-processing parts include the cornea, pupil, crystalline lens, retina, and finally the optic nerve itself. Each part of the eye has a specific task to help the brain receive signals that it can translate into usable visual input. Movement of the eye is controlled by a series of muscles that serve to direct the eye. The size of the pupil determines how much light enters the eye.
Light first enters the human eye through a transparent layer called the cornea. The cornea has no blood supply and receives oxygen directly from the air instead. It is shaped to begin refracting light waves toward the rest of the eye. A healthy cornea is slightly thicker on the edges than in the center, but if the cornea is misshapen due to disease or injury, light entering the eye is distorted.
The pupil is the next passage for visible light. A reflex called the pupillary light response changes the pupil's size reflexively according to how bright the light is. Once light passes through the cornea and the pupil, it goes through a transparent gel-like material — the acqueous humor — that further refracts light waves to reach the crystalline lens. The crystalline lens is a flexible structure that adjusts itself according to the distance or size of a desired source of visual input. This is unlike the cornea, which is fixed in its magnification.
The lens becomes thicker to focus on objects at shorter distances. It flattens to focus on further or smaller objects. People who undergo cataract surgery and receive an artificial lens do not have this advantage. An image seen through the lens is actually upside down and backwards at this point due to the nature of lightwaves. The brain is able to perceive this topsy-turvy image properly.
Light travels further into the human eye from the lens to the retina through another clear substance called the vitreous humor. Still more refraction takes place in this substance. The retina is mainly a collection of nerve sells, called photoreceptors, that are able to perceive a certain range of lightwaves of the electromagnetic spectrum.
Photoreceptors consist mainly of rods and cones. Rods work in dim light and can perceive black and white. Cones perceive color and work in brighter light. The retina also has photoreceptors that help the eye react to bright light. These rare photoreceptors are called photosensitive ganglion cells.
After processing by the retina, light reaches the optic nerve, which then sends the information to the brain. The brain is able to interpret these combinations of light waves so that the human consciousness can understand them. Specifically, the retina translates light into electrical signals and sends the signals all the way to the back of the brain. The human eye serves as a conduit for the passage and conversion of light energy, but it is the brain that really does the seeing.
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