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Have you ever seen this "eye chart" before? It is smaller than the real version of the chart that is used to test how well you can see. It was first developed in 1863. If you have 20/20 vision, it means that at a distance of 20 feet, you can read a certain line (labeled 20) on the chart and that your vision is normal. If you can only see the top line clearly (the one labeled 200), then you have 20/200 vision. This means that you must be 20 feet from the chart to see what most people can see at 200 feet. By the way, if someone's vision is 20/200 or worse, then they are legally blind.
People wear glasses for several reasons. People who have "fuzzy" vision when they look at distant objects are called nearsighted (myopic). In these people, the image is focused in front of the retina. The lens of glasses needed to correct vision in nearsighted people is concave. People who need glasses for reading and who have "fuzzy" vision for objects close to them are called farsighted (hyperopia). In farsighted people, objects are focused behind the retina. The lens of glasses needed to correct vision in farsighted people is convex. Astigmatism is another reason that you might have to wear glasses.
|Did you know?
||Approximately 160 million people in the US wear glasses or contact lenses. (Source: Walker, T.C. and Miller, R.K., 2001 Health Care Business Market Research Handbook, Fifth Edition, Norcross (GA): Richard K. Miller and Associates, Inc., 2001.)|
|They said it!
||"Spectacles. I don't use them. All I ask is a
large, fair type, a strong daylight or gas-light, and one yard of focal
distance, and my eyes are as good as ever."|
--- Oliver Wendell Holmes, The Autocrat of the Breakfast Table, 1858
Copyright © 1996-2011, Eric H. Chudler, University of Washington