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Glass is a uniform amorphous solid material, usually produced when the viscous molten material cools very rapidly to below its glass transition temperature, without sufficient time for a regular crystal lattice to form. more...
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The most familiar form of glass is the silica-based material used for windows, containers and decorative objects.
Glass is a biologically inactive material that can be formed with very smooth and impervious surfaces. Glass is, however, brittle and will break into sharp shards. These properties can be modified or changed with the addition of other compounds or heat treatment.
Common glass contains about 70-72 weight % of silicon dioxide (SiO2). The major raw material is sand (or \"quartz sand\") that contains almost 100% of crystalline silica in the form of quartz. Even though it is an almost pure quartz, it may still contain a small amount (< 1%) of iron oxides that would color the glass, so this sand is usually enriched in the factory to reduce the iron oxide amount to < 0.05%. Large natural single crystals of quartz are purer silicon dioxide, and they, upon crushing, are used for the high quality specialty glasses. At last, synthetic amorphous silica (practically 100% pure) is the raw material for the most expensive specialty glasses.
Properties and uses
The most obvious characteristic of ordinary glass is that it is transparent to visible light (not all glassy materials are). This transparency is due to an absence of electronic transition states in the range of visible light, and because ordinary glass is homogeneous on all length scales greater than about a wavelength of visible light. (Heterogeneities cause light to be scattered, breaking up any coherent image transmission). Ordinary glass partially blocks UVA (wavelength between 400 and 300 nm) and totally blocks UVC and UVB (wavelengths shorter than 300 nm) due to the addition of compounds such as soda ash (sodium carbonate).
Pure SiO2 glass (also called fused quartz) does not absorb UV light and is used for applications that require transparency in this region, although it is more expensive. This type of glass can be made so pure that, when made into fibre optic cables, hundreds of kilometres of glass are transparent at infrared wavelengths. Individual fibres are given an equally transparent core of SiO2/GeO2 glass, which has only slightly different optical properties (the germanium contributing to a higher index of refraction). Undersea cables have sections doped with erbium, which amplify transmitted signals by laser emission from within the glass itself. Amorphous SiO2 is also used as a dielectric material in integrated circuits, due to the smooth and electrically neutral interface it forms with silicon.
Glasses used for making optical devices are categorized using a six-digit glass code, or alternatively a letter-number code from the Schott Glass catalogue. For example, BK7 is a low-dispersion borosilicate crown glass, and SF10 is a high-dispersion dense flint glass. The glasses are arranged by composition, refractive index, and Abbe number.
Read more at Wikipedia.org
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