In approximately the year 1000, Italian glass blowers began making hand-held reading stones, much like today's magnifying glasses. The first actual pair of eyeglasses was not invented until nearly 300 years later, out of magnifying glasses and bone or leather. Since then, numerous strides have been made in fabricating eyeglasses.
Eyeglasses
Eyeglasses correct problems within the eye by refracting light through either a concave or a convex piece of plastic or glass. People may be nearsighted (myopic), which means they have trouble seeing objects that are far away; farsighted (hyperopic), meaning they have trouble seeing objects that are close up; have an astigmatism, which is an irregularly shaped cornea that can cause all objects to look blurry; or any combination of these.
Types of Lenses
Concave lenses are necessary to correct nearsightedness. These lenses are thinnest in the center. Convex lenses, which are thickest in the center, are used to correct farsightedness. Cylindrical lenses are used for people with astigmatism, and may curve in different directions, depending on the severity of the astigmatism. Bifocals lenses are usually concave toward the top and convex toward the bottom, for people who are nearsighted and farsighted. Progressive lenses are the same as bifocal lenses, but there is no line between the two types of lenses.
Lens Treatments
When ordering eyeglasses, a variety of treatments for the lenses are available depending on how you will use your glasses. Scratch resistance helps to keep your lenses usable longer, and UV protection protects eyes against the sun's rays. Anti-reflective coatings can make it easier to drive at night by decreasing the amount of glare from streetlights and other cars' headlights. Lenses can be tinted to function as sunglasses, or can be made from photochromic material that darkens when you walk in the sun.
