How the Human Eye Works

Ker Than

Date: 28 November 2005 Time: 02:51 AM ET

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.

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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.