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Monday, September 17, 2007

How the eye works

Summary





The human eye is remarkable. It accommodates to changing lighting conditions and focuses light rays originating from various distances from the eye. When all of the components of the eye function properly, light is converted to impulses and conveyed to the brain where an image is perceived.


Light rays enter the eye through a transparent layer of tissue known as the cornea.

As the eye's main focusing element, the cornea takes widely diverging rays of light and bends them through the pupil, the dark, round opening in the center of the colored iris.


The lens of the eye is located immediately behind the pupil. The purpose of the lens is to make the delicate adjustments in the path of the light rays in order to bring the light into focus upon the retina, the membrane containing photoreceptor nerve cells that lines the inside back wall of the eye.


The photoreceptor nerve cells of the retina change the light rays into electrical impulses and send them through the optic nerve to the brain where an image is perceived.
The individual components of the eye work in a manner similar to a camera. Each part plays a vital role in providing clear vision.


More detailed info


The human eye belongs to a general group of eyes found in nature called “camera-type eyes.” Instead of film, the human eye focuses light onto a light sensitive membrane called the retina.

Here’s how the human eye is put together and how it works:


The cornea is a transparent structure found in the very front of the eye that helps to focus incoming light. Behind the cornea is a colored ring-shaped membrane called the iris.


The iris has an adjustable circular opening called the pupil, which can expand or contract depending on the amount of light entering the eye.
A clear fluid called the aqueous humor fills the space between the cornea and the iris.
Situated behind the pupil is a colorless, transparent structure called the crystalline lens. Ciliary muscles surround the lens. The muscles hold the lens in place but they also play an important role in vision.

When the muscles relax, they pull on and flatten the lens, allowing the eye to see objects that are far away. To see closer objects clearly, the ciliary muscle must contract in order to thicken the lens.
The interior chamber of the eyeball is filled with a jelly-like tissue called the vitreous humor. After passing through the lens, light must travel through this humor before striking the sensitive layer of cells called the retina.
The retina is the innermost of three tissue layers that make up the eye. The outermost layer, called the sclera, is what gives most of the eyeball its white color. The cornea is also a part of outer layer.

The middle layer between the retina and sclera is called the choroid. The choroid contains blood vessels that supply the retina with nutrients and oxygen and removes its waste products.
Embedded in the retina are millions of light sensitive cells, which come in two main varieties: rods and cones.


Rods are good for monochrome vision in poor light, while cones are used for color and for the detection of fine detail. Cones are packed into a part of the retina directly behind the retina called the fovea.


When light strikes either the rods or the cones of the retina, it's converted into an electric signal that is relayed to the brain via the optic nerve. The brain then translates the electrical signals into the images we see.







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