The human eye is like a camera that collects, focuses, and transmits light through a lens to create an image of its surroundings. In a camera, the image is created on film or an image sensor. In the eye, the image is created on the retina, a thin layer of light-sensitive tissue at the back of the eye.
Like a camera, the human eye controls the amount of light that enters the eye. The iris (the colored circular part of the eye) controls the amount of light passing through the pupil. It closes up the pupil in bright light and opens it wider in dim light. The corneais the transparent, protective surface of the eye. It helps focus light, as does the lens, which sits just behind the iris.
When light enters the eye, the retina changes the light into nerve signals. The retina then sends these signals along the optic nerve(a cable of more than 1,000,000 nerve fibers) to the brain. Without a retina or optic nerve, the eye can’t communicate with the brain, making vision impossible.
What Is Visual Impairment?
Many people have some type of visual problem at some point in their lives. Some can no longer see objects far away. Others have problems reading small print. These types of conditions are often easily treated with eyeglasses or contact lenses.
But when one or more parts of the eye or brain that are needed to process images become diseased or damaged, severe or total loss of vision can occur. In these cases, vision can’t be fully restored or corrected to a “normal” level with medical treatment, surgery, or corrective lenses like glasses or contacts. It is a loss of vision that makes it hard or impossible to do daily tasks without specialized adaptations. Vision impairment may be caused by a loss of visual acuity, where the eye does not see objects as clearly as usual. It may also be caused by a loss of visual field, where the eye cannot see as wide an area as usual without moving the eyes or turning the head. Nearly two-thirds of children with vision impairment also have one or more other developmental disabilities, such as mental retardation, cerebral palsy, hearing loss, or epilepsy.
What Causes Visual Impairment?
People rarely lose their eyesight during their teen years. When they do, it’s usually caused by an injury like getting hit in the eye or head with a baseball or having an automobile or motorcycle accident.
Some babies have congenital blindness, which means they are visually impaired at birth. Congenital blindness can be caused by a number of things — it can be inherited, for instance, or caused by an infection (like German measles) that’s transmitted from the mother to the developing fetus during pregnancy.