How+Do+Eyes+See?

__**HOW DO EYES SEE?**__

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How the human eye can see begins when light rays are reflected off an object and enter the eyes through the cornea, the transparent outer covering of the eye. The cornea bends or refracts the rays that pass through a round hole called the pupil. The iris, or the coloured portion of the eye that is around the pupil, opens and closes (making the pupil bigger or smaller) to control the amount of light passing through. The light rays then pass through the lens, which actually changes shape so it can further bend the rays and focus them on the retina at the back of the eye. The retina is a thin layer of tissue at the back of the eye that contains millions of tiny light-sensing nerve cells called rods and cones, which are named for their distinct shapes. Cones are concentrated in the center of the retina, in an area called the macula. In bright light conditions, cones provide clear, sharp central vision and detect colors and fine details. Rods are located outside the macula and extend all the way to the outer edge of the retina. They provide peripheral or side vision. Rods also allow the eyes to detect motion and help us see in dim light and at night. These cells in the retina convert the light into electrical impulses. The optic nerve sends these impulses to the brain where an image is produced.

Megan Lee

Citation: http://www.aoa.org/x6024.xml

Article by: ©2006-11 American Optometric Association

__Additional Responses:__

How do eyes see? Eyes are small, yet complex organs and have many layer much like an onion. These Ping-Pong ball sized organs sit in the eye sockets surrounded with bones and muscles. The eyelids and tears protect them form things getting in the eye like dirt etc. Tears essentially wash out the eye when there is something in the eye that should not be. They also protect against infection. Muscles around eyes act like strings controlling a puppet. This is how your eyes move at the same time in the same direction.

Vision if a process where light and images are captured and interpreted by the brain. The sclera is the white part of your eye. It attaches tot eh cornea. Over top of the sclera lays the conjunctiva, a clear mucous membrane that protects your eyes and keeps them form drying out. The very inside of the eye is a clear gel like substance called the vitreous. It allows light to pass through to the retina. The cornea is a clear film that lies over top of the eye and focuses the light to the eye. Behind the cornea is the aqueous humor; together they flex light to the eye. This is where most of the focusing is done. The iris, the coloured part of your eye that controls how much light enters the pupil. The pupil is the black circle in the center of your eye. Your eyes are like cameras, they direct the amount of light to prevent under and over exposure. To do this the iris widens and narrows the pupil gets bigger when it needs more light and smaller when there is an abundance of light. The lens of the eye is directly behind the iris. The lens focused the light even further into sharp images.

To see distances the lens needs to change size. People who have normal vision their ciliary body in their eyes flattens so much so that objects are brought into focus from distances of 20 feet or more. To see things that re closer the muscle is compacted to the thickness of the lens. As you age the ciliary body becomes less elastic because of this a large number of people over the age of 45 have to hold things like books further away to see them properly. The retina is a sensitive layer of tissue that is at the back of the eyeball wall. It is made up of millions of light receptors that are called rods and cones. Rods are extremely sensitive to light in comparison. There is about 125 million rods in each eye. This is so that you can see dimed light and shade of gray, but rods cannot see colours. Cones on the other hand there are 6 million cones in each eye. Cones allow us to distinguish vibrant light and colours with great detail. The macula is a tiny specialized area on the retina that helps us see very small details when we look at something directly. This part contains very few rods and a lot of cones. When light is shone onto the retina and simulates the rods and cones. The retina sends signals to the back of the eye into the optic nerve which then carries these signals to the brain. The brain then interprets these signals as visual images. Just like a camera our eyes’ lenses see things upside down. The brain learns to receive impulses form the upper part of the retina are truly the lower part of the thing we are looking at so we seeing and vice versa.

Citation: "kids health ." nemours. [] Author unknown

Beth Durnie

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