Morton Tavel, a professor of physics at Vassar College, responds:
"The propagation of light (or any other form of electromagnetic radiation) through a solid is a complex process that involves not just the passage of the incident light but also reradiation of that light by the electronic structure of the solid. The convoluted combination of reflection and transmission explains why light moves more slowly through solids than through the air or through a vacuum.
"Simply stated, a solid material will appear transparent if there are no processes that compete with transmission, either by absorbing the light or by scattering it in other directions. In pure silicon, there is a very strong absorptive process at work: the incident visible light is absorbed by electrons that then move from one electron energy state to another (an occurrence technically known as a band-to-band transition). Glass, being silicon dioxide--not pure silicon--does not have this band structure, so it cannot absorb light as pure silicon does. Sand, on the other hand, is also silicon dioxide, but it is so filled with impurities that light simply scatters outward incoherently and does not pass through to a noticeable extent.
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"The electronic structure of solids also explains why metals are shiny. Pure metals reflect light but do not transmit it, because they are filled with free electrons. These electrons reradiate the light in the direction opposite from which it arrived (reflection), but they interfere with the light that would proceed in the forward direction, preventing transmission."
Susan Murphree Thomas is a researcher in inorganic chemistry at the Georgia Institute of Technology and an interim faculty member at Kennesaw State University. She adds some details about the role of physical structure:
"A material appears transparent when it does not strongly absorb or diffract light. As far as the absorbance of a solid goes, you pretty much have to take what Nature gives you. Diffraction, however, can be influenced by how the material is prepared.
"A material that appears homogeneous to the human eye is really made up of minute crystals--regions in which the atoms or molecules follow a regular order. The boundaries between these regions are called grain boundaries. If the distance between boundaries is smaller than the shortest wavelength of visible light (in other words, if the refractive index of the material is uniform with respect to the light passing through it), then the material will appear transparent. Each boundary tends to diffuse the light that passes through; if the regions are small enough, however, the light waves essentially 'jump' right over them.
"Glass (which consists of silicon dioxide along with a few impurities) is not really a solid; it can be more accurately thought of as a supercooled liquid. It has no internal grain boundaries, and hence it looks transparent. Solid silicon dioxide (sand), in contrast, has obvious grain boundaries, so it is not transparent.
"It is possible to create an artificially uniform material. One way to do this is to press a material under force, as is done all the time with potassium bromide, a compound used for infrared spectroscopy in laboratories. The other way to achieve uniformity is to create lots of nucleation sites (the locations where crystals begin to form) in a melted material and then allow it to cool. Because many little crystals begin to form all at once, none of them can grow very large before they run into one another. The transparent Corningware sold today is made in this manner. It has the transparency of glass, but it is really a ceramic material similar to regular Corningware.
