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Optical properties
Perhaps the most peculiar property of glass is its transparency. It may look paradoxical, but glass is "crystal-clear" just because it is not crystalline.

Crystalline materials which constitute most minerals, infact, are very rarely perfect single crystals (monocrystals). Usually they have a "granular" structure, because they are polycrystalline: a white marble can be described as a 3-D mosaic of microscopic crystals (crystallites) held together. The single crystallites may be transparent and clear, but they have facets (grain boundaries), which can reflect or scatter light. So, light entering into the stone is repeatedly scattered until it re-emerges form the surface in random directions. This subsurface scattering mechanism, together with scattering by surface irregularities, gives rise to diffuse reflection.

The result of diffuse reflection is that marble reflects back nearly all the light it receives, and, though it does not absorb light, is not transparent, but white, in spite of the fact that it is made of transparent objects (crystallites). This mechanism, which makes objects to be opaque, is common to most materials, inorganic and organic (in this case the scattering centers are the cells or the fibers constituting them) and is a crucial mechanism for vision, because most obiects are seen by our eyes through their diffuse reflection, which hence is also important in 3D computer graphics, for the realistic rendering of material surfaces.

Glass, instead, (like liquids) has no such internal subdivisions. The "disorders" due to its amorphous structure have a spatial scale much smaller than the wavelength of light, so that light "does not see" them, and has nearly zero scattering. The surface, too, is naturally smooth, because the molecules, when solidifying, are not forced to dispose in rigid crystal geometries, and can follow surface tension, which imposes a microscopically smooth surface. These properties, which give glass its clearness, can be retained even if glass is partially absorbing (colored, see below). So, glass can imitate perfect monocrystalline gems with polished surfaces and is indeed crystal clear, which is a property shared with liquids, but very rare in nature in solid objects.

So glass can refract, reflect and transmit light following geometrical optics, without scattering it, and we can use it to make lenses and windows. Common glass has a refraction index around 1.5. According to Fresnel equations, the reflectivity of a sheet of glass is about 4% per each surface (at normal incidence), and its transmissivity about 92%.