The primary function of the human pupil is to regulate the amount of light that enters the eye. When the pupil is smaller, less light enters the eye in bright environments. The pupil gets larger as light levels decrease to let more light into the eye. Doctors examine the pupil to ascertain one's neurological function.
Surrounding the pupil is the iris, the colored portion of the eye. The dilator and sphincter muscles in the iris control how wide the pupil gets. These muscles expand and contract to widen or shrink the aperture. Light enters the pupil, goes through the lens and focuses on the back of the eye.
Covering the pupil is the cornea, the clear outer layer of the eye that protects the pupil. After light hits the lens, the image is focused and the retina converts light into electrical signals interpreted by the brain due to the optic nerve.
Several conditions affect the amount of light gathered by the pupil. A corneal abrasion is a scratch on the front part of the eye that may cause light sensitivity. Myopia, or nearsightedness, prevents light from focusing properly at the back of the eye. Hyphema is a condition marked by bleeding in between the pupil and cornea. Hyphema requires urgent medical attention and surgery to correct the problem.