How The Eye Works

The CORNEA

The cornea is the transparent window at the front of the eye that allows light to pass into the eye.

The PUPIL

The pupil is like the aperture of a camera. Light enters here, then passes through the lens. Liquid flowing through the pupil, called aqueous fluid, is absorbed through channels in front of the iris. This fluid provides nourishment to the lens, iris and cornea. Pressure within the eye depends on the delicate balance between the body's production and absorption of this fluid.

The IRIS

With a camera, you can enlarge or reduce the aperture to control the amount of light that enters. Similarly, the iris—the colored portion of the eye, automatically opens (dilates) or closes (constricts) the pupil to adapt to the surrounding light.

The LENS

The lens in the healthy eye is flexible or deformable. The ring-shaped ciliary muscle surrounding it contracts or relaxes to adjust its shape according to visual need. The lens bends or "refracts" light rays to focus on near or far objects. The light rays pass through the transparent gel that fills the inside of the eyeball—the vitreous gel, and come to a focus on the surface of the retina.

The RETINA

The retina is a light-sensitive tissue that lines the back of the eye, comparable to film in a camera. The retina receives the picture formed by the light rays and sends the image to the brain through a cable known as the optic nerve.

The MACULA

The central part of the retina, which processes detailed vision, is called the macula. It is here that the optic nerve collects and transmits visual images to the brain for processing. The brain combines the separate images produced by the eyes into a single, composite neural image.

The VITREOUS

The vitreous is the clear, jello-like substance that fills the hollow center of the eye.

The OPTIC NERVE

The optic nerve is the cable that transmits the image from the eye to the brain.

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Frequently Asked Questions

The eye is a spherical structure about an inch in diameter. It has a clear bulge on the front side, which is the cornea. The wall of the eye beyond the cornea consists of three tissue layers. The outermost layer is the sclera, a tough, protective coating that covers most of the outer surface of the eye that connects to the transparent cornea at the front of the eye. The middle layer is the choroid, a vascular layer that is continuous with the ciliary body and the iris on the front side of the eye. The inner layer is the retina, a light-sensitive tissue that lines the inside back wall of the eye. To understand how the eye works, it is useful to think of a camera. The human eye is more complex than a television camera, but the principles are similar. Place your cursor on the eye's parts below to see how they function.