How the Human Eye Works
The human eye can be compared to a camera which gathers, focuses, and transmits light through a lens to create an image of the environment. In a camera, the image is created on film; in the eye, the image is created on the retina, a thin layer of light sensitive cells at the back of the eye. The lens of the eye bends, or refracts, light that enters the eye.
The cornea, which is a clear, transparent covering in the front portion of the eye also contributes to focusing light on the retina. Nerve fibers extending back from the retina's nerve cells come together behind the retina to form the optic nerve, a "cable" of nerve fibers connecting the eye with the brain.
The optic nerve transmits messages about what we see from the eye to the brain. Like a camera, the human eye controls the amount of light that enters the eye through the lens under various lighting conditions.
Visual Acuity and 20/20 Vision
Visual acuity is the sharpness of vision determined by a person's ability to discriminate fine details, and is measured by using specially devised tests and charts. One chart that is commonly used for measuring visual acuity is the Snellen chart, which contains letters of the alphabet arranged by line, with each line of letters from the bottom up increasing in size. The letters on the lowest line are the smallest letters on the chart, and the letter at the top is the largest. The character on the bottom line represent 20/20 vision; the single large letter at the top represents 20/400. The designation of legal blindness is 20/200.
When the Snellen chart is used, visual acuity is generally measured with a person seated 20 feet away from the chart. A person who has normal visual acuity has 20/20 vision. This means that at 20 feet the person can see the line of letters that people with normal sight see from 20 feet.
Why People Need Glasses
Most often people need to wear eyeglasses to correct blurred or distorted vision caused by imperfections in the eyes' focusing mechanism. These imperfections, which occur because light entering the eye is not brought into sharp focus on the retina, are known as common errors of refraction or refractive errors.
Refractive errors occur as a result of irregularities in the shape of the cornea, the actual size or shape of the eyeball itself, or the focusing capacity of the lens. Common refractive errors that are fully corrected with eyeglasses or contact lenses are not visual impairments because sight can be corrected to normal. Nearly every person is likely to have a refractive error at some point in life, especially after age 40, and perhaps need to wear eyeglasses or contact lenses. The common refractive errors are:
Visual Impairment
Visual impairment describes vision that cannot be fully corrected by ordinary prescription lenses, medical treatment, or surgery. The term visual impairment includes conditions ranging from the presence of good usable vision, low vision, or to the absence of any sight at all--total blindness. Many terms are used when people refer to visual impairment. These terms are explained below.