The eye is a complex organ that works much like a camera, focusing light rays and forming an image. On the surface of the eye is the cornea, a thin, spherical layer of tissue that provides a clear window for light to pass through. In a healthy eye, the cornea bends or refracts light rays so they focus precisely on the retina in the back of the eye.
Behind the cornea is the iris, the colored part of the eye we refer to when we say a person has brown or blue eyes. In the center of the iris is the pupil. The iris functions like a shutter, adjusting pupil size to control the amount of light entering the eye.
Located behind the iris is the lens, which works together with the cornea and vitreous to focus light. Like the lens in a camera, it adjusts light rays as vision shifts between nearby and distant objects in a process called accommodation.
Light then passes through the vitreous, the watery gelatinous substance that fills most of the eye.
The back of the eye is lined with a thin layer of tissue containing millions of photoreceptor (light-sensitive) cells. This is the retina, where light rays focus into an upside-down image. In the center of the retina is the macula. Less than 1/4 of an inch in diameter, the macula is responsible for clear central vision. The retina converts the image into an electrical signal that travels down the optic nerve to the brain.