Basic Eye Functions

The process of seeing images begins when light enters the eye through the pupil and becomes focused by the lens onto the retina. Light receptors called rods and cones convert light to electrical signals. The optic nerve transmits these electrical signals to the brain where they are interpreted as images.

Lens

• The lens is transparent and focuses light onto retina photoreceptors, according to the University of Michigan Kellogg Eye Center. As people age, the lens of the eye deteriorates, but can be replaced medically with phakic intraocular lenses, according to the United States Food and Drug Administration.

Pupil

• The pupil changes size as it adjusts to the amount of light available to the eye, according to the University of Michigan Kellogg Eye Center. If too much light hits the eye, damage to the retina can occur. If too little light is available, eyesight becomes difficult. The pupil becomes smaller when more light is available and larger when less light is available. The size of the pupil is controlled by the iris. The pupil is an opening light passes through as it enters the eye.

Iris

• The iris is the colored circular muscle that surrounds the black pupil, according to the University of Utah. The iris controls the size of the pupil by relaxing and contracting. When the iris relaxes, the pupil becomes larger, and when it contracts, the pupil becomes smaller, according to the University of Pennsylvania Medical Center.

Sclera

• The sclera is the white outer coat of the eye that surrounds the iris, according to the University of Michigan Kellogg Eye Center. The sclera is a tough, fibrous tissue that extends from the cornea to the optic nerve in the back of the eye, according to MedlinePlus. The sclera provides attachment for the intrinsic muscles of the eye.

Retina

• The retina converts focused light from the lens into electrical signals before sending them to the brain, according to the University of Washington. Rods and cones are the scientific terms for retina photoreceptors. The retina contains 125 million rods and between 6 and 7 million cones. Rods are used for seeing dim light, and cones are used for seeing bright, sharp images and colors. The macula is the part of the retina that contains light-sensitive cells, according to the University of Michigan Kellogg Eye Center. The fovea is the center of the macula and helps with sharp vision.

Optic Nerve

• The optic nerve delivers electrical signals from the retina to the visual cortex of the brain, according to The Ohio State University Medical Center. The optic nerve contains more than one million nerve fibers, according to the OHSU Casey Eye Institute.

Choroid

• The choroid is a layer of blood vessels located between the retina and sclera, according to the University of Michigan Kellogg Eye Center. The choroid provides the outer layers of the retina with oxygen and nutrients.

Vitreous

• The vitreous humor is a clear gel that provides constant pressure to maintain the eye's shape, according to the University of Washington.