B. Structure of the eye

The eye can be considered as a complete optical system, including packaging, a variable aperture, a curved corneal surface and a lens that provide for imaging, a variable focus capability, a photosensor, and an output to a computer, the brain. Figure 6-16 shows a simplified diagram of the structure of the eye. The eye is approximately spherical in shape and is contained within a tough, fibrous envelope of tissue called the sclera. The sclera covers all the eyeball except for a portion of its front. At the front of the eyeball is the cornea, which has a refractive index around 1.38. The cornea is a transparent membrane that allows light to enter the eyeball and that contributes significantly to the focusing capability of the eye. Behind the cornea is the iris, an adjustable aperture that expands in dim light and contracts in bright light, controlling the amount of light which enters the eyeball. The pupil of the eye is the opening in the center of the aperture defined by the iris. Light entering the eye passes through the pupil.

Figure 6-16 Structure of the human eye

The region behind the cornea contains a transparent liquid called the aqueous humor with refractive index around 1.34. Then there is the lens of the eye, a capsule of fibrous jelly-like material, with refractive index varying from 1.41 in the center to 1.39 at the periphery. The shape of the lens can be changed by muscles attached to it. This allows for fine focusing of light entering the eye.

After the lens is a transparent thin jelly called the vitreous humor. It has a refractive index around 1.34. Finally, covering most of the back surface of the eyeball is the retina, the photosensitive medium that serves as the actual detector material.

The cells in the retina are of two types, called rods and cones. The rods and cones serve different functions, the cones providing the most sensitive vision near the center of the retina and the rods the peripheral vision farther out in the retina. The rods also are more sensitive in dim light than are the cones, so that the rods tend to dominate night vision.

Near the center of the retina is a slight depression, called the fovea centralis, that contains only cones. This region provides the most acute vision.

The rods and cones are connected through nerve fibers to the optic nerve, which emerges from the back of the eyeball. The rods and cones receive the optical image and transmit it through the nerve fibers to the brain. At the point where the optic nerve exits the eyeball, there are no rods or cones, so there is a small blind spot at that position.