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Glasses

From Academic Kids

Image:Eyeglasses.jpg

Glasses, spectacles, or eyeglasses are frames bearing lenses worn in front of the eyes, sometimes for purely aesthetic reasons but normally for vision correction or eye protection. Special glasses are used for viewing 3-D images or virtual reality.

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 are more often called eyeglasses in North American English, spectacles in British English, and (rarely) frames or lenses. Spectacles is often shortened to specs. In hipster slang they are cheaters. Also see usage of words for eyepieces for a more detailed examination of the different usages for these words.

Glasses were originally made from glass, but many are now made from plastic due to the danger of breakage and the greater weight of glass lenses. Scratch-resistant coatings can be applied to most plastic lenses giving them similar scratch resistance to glass. Also, a hydrophobic coating can be applied to make lenses easy to clean.

Corrective spectacles have lenses shaped to correct vision abnormalities.

Safety glasses are a kind of eye protection against flying debris or visible and near visible light or radiation.

Sunglasses protect against high levels of visible and ultraviolet light.

Contents

History

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Detail of a portrait of Hugh de Provence, painted by Tomasso da Modena in 1352

Glasses were invented in northern Italy, most likely in the late 1280s. The identity of the original inventor is unknown. In 1676, Franciscus Redi, a professor of medicine at the University of Pisa, wrote that he possessed a 1289 manuscript whose author complains that he would be unable to read or write were it not for the recent invention of glasses, and a record of a sermon given in 1305, in which the speaker, a Dominican monk named Fra Giordano da Rivalto, remarked that glasses had been invented less than twenty years previously, and that he had met the inventor. Based on this evidence, Redi credited another Dominican monk, Fra Alessandro da Spina of Pisa, with the re-invention of glasses after their original inventor kept them a secret, a claim contained in da Spina's obituary record. In 1738, a Florentine historian named Domenico Manni reported that a tombstone in Florence credited one Salvino d'Armato (died 1317) with the invention of glasses. Other stories, possibly legendary, credit Roger Bacon with the invention. Bacon's published writings describe the magnifying glass (which he did not invent), but make no mention of glasses.

These early spectacles had convex lenses that could correct the farsightedness (presbyopia) that commonly develops as a symptom of aging. Nicholas of Cusa is believed to have discovered the benefits of concave lens in the treatment of myopia (nearsightedness). However, it was not until 1604 that Johannes Kepler published in his treatise on optics and astronomy, the first correct explanation as to why convex and concave lenses could correct presbyopia and myopia. The American scientist Benjamin Franklin, who suffered from both myopia and presbyopia, invented bifocals in 1784 to avoid having to regularly switch between two pairs of glasses. The first lenses for correcting astigmatism were constructed by the British astronomer George Airy in 1827.

Over time, the construction of spectacle frames also evolved. Early eyepieces were designed to be either held in place by hand or by exerting pressure on the nose (pince-nez). Girolamo Savonarola suggested that eyepieces could be held in place by a ribbon passed over the wearer's head, this in turn secured by the weight of a hat. The modern style of glasses, held by temples passing over the ears, was developed in 1727 by the British optician Edward Scarlett. These designs were not immediately successful, however, and various styles with attached handles such as scissors glasses and lorgnettes remained fashionable throughout the 18th and into the early 19th century.

Corrective glasses

Corrective lenses modify the focal length of the eye's pupil to alleviate the effects of shortsightedness (myopia), longsightedness (hyperopia) or astigmatism. In North America lenses made to conform to the prescription of an Ophthalmologist or Optometrist are called prescription lenses and are used to make prescription glasses.

Safety glasses

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SafetyGlassesShield.jpg
Safety glasses with side shields
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SafetyGlassesWraparound.jpg
Wraparound safety glasses

Safety glasses are usually made with shatter-resistant plastic lenses to protect the eye from flying debris. Safety glasses can vary in the level of protection they provide. For example, those used in medicine may be expected to protect only against blood splatter while safety glasses in a factory might have stronger lenses and a stronger frame with additional shields at the temples. The lenses of safety glasses can also be shaped for correction or tinted to protect against visible and near-visible radiation. Some safety glasses are designed to fit over corrective glasses or sunglasses. They may provide less eye protection than goggles, face shields or other forms of eye protection, but their light weight increases the likelihood that they will actually be used. Recent safety glasses have tended to be given a more stylish design, in order to encourage their use. The pictured wraparound safety glasses are evidence of this style change with the close fitting nature of the wraparound dispensing with the need for side shields.

Corrective glasses with plastic lenses can often be used in the place of safety glasses in many environments; this is one advantage that they have over contact lenses.


Sunglasses

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Scratch-resistant sunglasses made using a NASA developed coating

Main article: Sunglasses

Sunglasses are darkened glasses that provide protection against bright visible and ultraviolet light. Due to changes in the atmosphere, ultraviolet levels are much higher than in the past and ultraviolet protection for eyes and skin is even more important. Sunglasses vary greatly and many offer more style than protection. It is possible to have lenses that look very dark and yet offer little ultraviolet protection.

Prescription lenses may be used to make prescription sunglasses. Glasses with photo-sensitive lenses, called transition lenses, become darker in the presence of UV light. Unfortunately, many car windshields protect the passengers from UV light, while not shielding from bright visible light, making transition lenses ineffective where they are most needed. On the other hand, they will presumably darken in the presence of black lights, which do not create large amounts of visible light, and, in fact, are usually used in dark places, making the lenses effective when the user would probably not want them to be.

Special glasses

The illusion of three dimensions on a two dimensional surface can be created by providing each eye with different visual information. Classic 3-D glasses create the illusion of three dimensions when viewing specially prepared images. The classic 3-D glasses have one red lens and one blue lens. 3-D glasses made of cardboard and plastic are distributed at 3-D movies. Another kind of 3-D glasses uses polarized filters. One kind of electronic 3-D spectacles uses electronic shutters.

Virtual reality glasses and helmets have separate video screens for each eye and a method for determining the direction the head is turned.

Conditions glasses are used to correct

Glasses fitted with corrective lenses are a common means of correcting focus problems such as myopia (nearsightedness, short-sightedness) and hyperopia (farsightedness, long-sightedness). Emetropia, the condition of ideal focus is described as two parallel rays of light entering the eye and converging on or at the retina. Myopia is a condition in which parallel rays of light entering the eye converge before reaching to retina. Hyperopia is a condition in which parallel rays of light entering the eye do not converge prior to reaching the retina. Some would say "the rays of light converge at an imaginary point behind the retina." Astigmatism is a condition in which parallel rays of light entering the eye converge at two separate locations, either before and/or after the retina. As most people age the crystalline lens of the eye loses elasticity resulting in presbyopia, which limits their ability to focus on nearby objects. None of these conditions is considered a disease.

Variation in glasses

Glasses can be very simple. Magnifying lenses for reading that are used to treat mild hypermetropia and presbyopia can be bought off the shelf, but most glasses are made to a particular prescription, based on degree of myopia or hypermetropia combined with astigmatism. Lenses can be ground to specific eyes, but in most cases standard off-the-shelf prescriptions suffice, but require custom-fitting to particular frames.

As people age, their ability to focus is lessened and many decide to use multiple-focus lenses, bifocal or even trifocal to cover all the situations in which they use their sight. Traditional multifocal lenses have two or three distinct viewing areas, each requiring a conscious effort of refocusing. These were originally separate lenses, as invented by Benjamin Franklin. Some modern multifocal lenses give a smooth transition between these lenses, unnoticeable by most wearers. Other spectacle wearers sometimes have lenses specifically intended for use with computer monitors at a fixed distance. On the other hand, many people simply have several pairs of glasses, one for each task or distance, with specific glasses for reading, computer use, and television watching.

Glasses as a fashion accessory

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Two women wearing designer glasses

Glasses are often regarded as unattractive, and many people prefer to wear contact lenses for that reason. Contact lenses also provide much improved peripheral vision.

On the other hand, many people are attracted to people who wear glasses, and glasses are available in a wide range of styles, materials, and even designer labels.

Glasses can be a major part of personal expression, from the extravagance of Elton John and Dame Edna Everage, from Groucho Marx to John Denver to Lisa Loeb all the way to the varied professional personas of eyeglass-wearing knowledge workers.

For some celebrities, glasses form part of the identity. American Senator Barry Goldwater continued to wear lensless horn-rimmed spectacles after being fitted with contact lenses because he was not recognizable without his trademark glasses. British soap star Anne Kirkbride had the same problem: her character on Coronation Street, Deirdre Barlow, became so well-known for her big frames that she was expected to wear them at social gatherings and in international tours, even though Kirkbride has always worn contact lenses. Drew Carey continued to wear glasses for the same reason after getting corrective laser eye surgery. British comedic actor Eric Sykes, who became profoundly deaf as an adult, wears glasses that contain no lenses, but are in fact a bone-conducting hearing aid. Masaharu Morimoto wears glasses to separate his professional persona as a chef from his stage persona as Iron Chef Japanese. And of course John Lennon wore his round "granny glasses" from some of his time with the Beatles to his assassination in 1980.

In popular culture, glasses were all the disguise Superman, Spider-Man, and Wonder Woman needed to hide in plain view as alter egos Clark Kent, Peter Parker, Diana Prince, respectively.

An example of halo effect is seen in the stereotype that those who wear glasses are intelligent or, especially in teen culture, even geeks. This conception probably comes from an era when most people were illiterate and the first people to wear glasses were those who did a lot of reading. Some people who find that wearing glasses may look nerdy turn to contact lenses instead.

Another unpopular aspect of glasses is their inconvenience. Even though the late-20th century saw the creation of light frames, such as those of made of titanium, very flexible frames, and new lens materials and coatings, glasses can still cause problems during rigorous sports. The lenses themselves can also become greasy or trap vapour when eating hot food or swimming or walking in rain, reducing visibility significantly. Scraping, fracturing, or breakage of the lenses require time-consuming and costly professional repair, though modern plastic lenses are almost indestructable and very scratch-resistant.

See also

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External Links

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