Sunglasses

Sunglasses or sun glasses are a form of protective eyewear that usually enclose or protect the eye pupil in order to prevent strong light, ultraviolet (UV) rays, and increasingly, blue light ("blue blocking") from penetrating. They can sometimes also function as a visual aid, as variously termed spectacles or glasses exist which feature lenses that are colored, polarized or darkened. In the early 20th century they were also known as sun cheaters (cheaters being an American slang term for glasses).[1]

Many people find direct sunlight too bright for comfort. During outdoor activities, the human eye can receive more light than usual. Healthcare professionals recommend eye protection whenever outside to protect the eyes from ultraviolet radiation and blue light, which can cause several serious eye problems. Sunglasses have long been associated with celebrities and film actors primarily from a desire to hide or mask their identity. Since the 1940s sunglasses have been popular as a fashion accessory, especially on the beach.

Peoria (named after the Peoria tribe) is the largest city on the Illinois River and the county seat of Peoria County,[1] Illinois, in the United States. As of the 2000 census, the city was the fifth-largest in Illinois, with a population of 112,936; by 2007 it was the sixth-largest city and had population of 113,546.[2] The Peoria Metropolitan Statistical Area had a population of 372,487 in 2008, making it the third largest metropolitan area in the state after Chicagoland and the Metro-East portion of the St. Louis metropolitan area.

Photochromic lenses are lenses that darken on exposure to ultraviolet (UV) radiation. Once the UV is removed (for example by walking indoors), the lenses will gradually return to their clear state. Photochromic lenses may be made of glass, polycarbonate, or another plastic. The glass version of this type of lenses was first developed by Corning in the 1960s. More recently, plastic versions of these lenses have been commercialized. The first of these was the Photolite lens sold in the early 1980s by American Optical Corporation. But the first commercially successful plastic photochromic lens was introduced by Transitions Optical in 1991.[1]

The glass version of these lenses achieve their photochromic properties through the embedding of microcrystalline silver halides (usually silver chloride), or molecules in a glass substrate. Plastic photochromic lenses rely on organic photochromic molecules (for example oxazines and naphthopyrans) to achieve the reversible darkening effect. The reason these lenses darken in sunlight but not indoors under artificial light, is that room light does not contain the UV (short wavelength light) found in sunlight. Automobile windows also block UV so these lenses would darken less in a car. Lenses that darken in response to visible (rather than UV) light would avoid these issues, but they are not feasible for most applications. In order to respond to light, it is necessary to absorb it, thus the glass could not be made to be clear in its low-light state. This correctly implies photochromic lenses are not entirely transparent, specifically they filter out UV light. This does not represent a problem, because the human eye does not see in the UV spectrum.