Glass can be made form transparent and flat, or into added shapes and colors as made known in this ball from the Verrerie of Brehat in Brittany. One of the nearly all obvious characteristics of ordinary glass is that it is clear to visible light. The clearness is due to an absence of electronic transition states in the range of visible light, and to the truth that such glass is homogeneous on all length scales greater than about a wavelength of noticeable light. Ordinary glass does not let light at a wavelength of lower than 400 nm, also recognized as ultraviolet light or UV, to pass. This is due to the addition of compounds for instance soda ash (sodium carbonate).
Pure SiO2 glass (also called fused quartz) does not absorb UV light and is used for applications that necessitate transparency in this region, although it is more costly. This kind of glass can be made so pure that hundreds of kilometers of glass are clear at infrared wavelengths in fiber optic cables. Individual fibers are given a uniformly transparent cladding of SiO2/GeO2 glass, which has only somewhat different optical properties (the germanium causative to a lower index of refraction). Undersea cables have sections doped with Erbium, which intensify transmitted signals by laser release from within the glass itself.
Amorphous SiO2 is also used as a dielectric substance in integrated circuits, owing to the smooth and electrically unbiased interface it forms with silicon. Glasses used for making visual devices are commonly categorized by means of a letter-number code from the Schott Glass catalog. For model, BK7 is a low-dispersion borosilicate crown glass, and SF10 is a high-dispersion opaque flint glass. The glasses are placed by composition, refractive indicator, and Abe number.
Glass is sometimes created obviously from volcanic magma. This glass is called obsidian, and is generally black with impurities. Obsidian is a raw substance for flint knappers, who have used it to make particularly sharp knives since the Stone Age. Obsidian collection is prohibited by law in some places (together with the United States), but the same tool making techniques can be useful to industrially-made glass.