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.
Light rays do not focus on the retina clearly.
The retina cannot sense light rays normally.
Nerve impulses from the retina are not transmitted to the brain normally.
The brain cannot interpret information sent by the eye.