EYEGLASSES

Glasses, spectacles, or eyeglasses are frames-bearing lenses worn in front of the eyes normally for vision correction or eye protection or for protection from UV rays. Special glasses are used for viewing three-dimensional images from two-dimensional displays or experiencing virtual reality. Eyeglasses were invented in Italy in the late 13th century.[1]

Modern glasses are typically supported by pads on the bridge of the nose and by temples placed over the ears. Historical types include the pince-nez, monocle, and lorgnette.

Glasses frames are commonly made from metal, horn or plastic. Lenses were originally made from glass, but many are now made from various types of plastic, including CR-39 or polycarbonate. These materials reduce the danger of breakage and weigh less than glass lenses. Some plastics also have more advantageous optical properties than glass, such as better transmission of visible light and greater absorption of ultraviolet light. Some plastics have a greater index of refraction than most types of glass; this is useful in the making of corrective lenses shaped to correct vision abnormalities such as myopia, allowing thinner lenses for a given prescription. Scratch-resistant coatings can be applied to most plastic lenses giving them similar scratch resistance to glass. Hydrophobic coatings designed to ease cleaning are also available, as are anti-reflective coatings intended to improve night vision and make the wearer's eyes more visible.

Polycarbonate lenses are the lightest and most shatterproof, making them the best for impact protection. Polycarbonate lenses offer poor optics because of a low Abbe number of 31. CR-39 lenses are the most common plastic lenses, due to their low weight, high scratch resistance and low transparency for ultra violet and infared radiation.

Some glasses are not designed for vision correction. Safety glasses are a kind of eye protection against flying debris or against visible and near visible light or radiation. Sunglasses allow better vision in bright daylight, and may protect against damage from high levels of ultraviolet light. Some glasses are also designed purely for aesthetic values, or for fashion values.

Corrective lenses modify the focal length of the eye to alleviate the effects of nearsightedness (myopia), farsightedness (hyperopia) or astigmatism. As people age, the eye's crystalline lens loses elasticity, resulting in presbyopia, which limits their ability to change focus.

The power of a lens is generally measured in diopters. Over-the-counter reading glasses are typically rated at +1.00 to +3.00 diopters. Glasses correcting for myopia will have negative diopter strengths. Lenses made to conform to the prescription of an ophthalmologist or optometrist are called prescription lenses and are used to make prescription glasses.

Usually:

• the "spherical" component is the main correction
• the "cylindrical" component is "fine tuning."

Depending on the optical setup, lenses can act as magnifiers, lenses can introduce blur, and lenses can correct blur.

Whatever the setup, spherical lenses act equally in all meridians: they magnify, introduce blur, or correct blur the same amount in every direction.

An ordinary magnifying glass is a kind of spherical lens. When a spherical lens acts as a magnifier, it magnifies equally in all meridians. Here, note that the magnified letters are magnified both in height and in width.

Some kinds of magnifying glasses, made specifically for reading wide columns of print, are cylindrical lenses. When a cylindrical lens acts as a magnifier, it magnifies only in one direction. For example, the magnifier shown magnifies letters only in height, not in width