Anything that blocks the passage of light from the environment to the back of the eye or disrupts the transmission of nerve impulses from the back of the eye to the brain will interfere with vision. In the United States, legal blindness is defined as a visual acuity of 20/200 or worse in the better eye, even after correction with eyeglasses or contact lenses, or a visual field restricted to less than 20° in the better eye. Many people who are considered legally blind can distinguish shapes and shadows but not normal detail.

Blindness can occur under the following circumstances:

Light cannot reach the retina.

• Damage to the cornea caused by infections such as herpes keratoconjunctivitis or an infection that follows contact lens overwearing, which results in an opaque corneal scar

• Damage to the cornea caused by vitamin A deficiency (keratomalacia), which causes dry eyes and results in an opaque corneal scar (rare in developed nations)

• Damage to the cornea caused by a severe injury that results in an opaque corneal scar

• A cataract, which causes loss of clarity of the lens

Light rays do not focus on the retina clearly.

• Imperfect focusing of light rays on the retina (refraction errors) that cannot be fully corrected with eyeglasses or contact lenses (such as from certain types of cataracts)

The retina cannot sense light rays normally.

• Detached retina

• Diabetes mellitus

• Macular degeneration

• Retinitis pigmentosa

• Inadequate blood supply to the retina, usually due to a blockage of the retinal artery or vein, which may be caused by inflammation of the blood vessel wall (such as that caused by giant cell arteritis), or due to a blood clot that travels to the eye from somewhere else (such as from the carotid artery in the neck)

• Infection of the retina (such as from Toxoplasma or fungi)

Nerve impulses from the retina are not transmitted to the brain normally.

• Disorders affecting the optic nerve or its pathways inside the brain, such as brain tumors, strokes, infections, and multiple sclerosis

• Glaucoma

• Inflammation of the optic nerve (optic neuritis)

The brain cannot interpret information sent by the eye.

• Disorders that affect the areas of the brain that interpret visual impulses (visual cortex), such as strokes and tumors