![]() ![]() Microstructure: scattering centers include internal surfaces such as grain boundaries, crystallographic defects and microscopic pores.Glassy structure: scattering centers include fluctuations in density or composition.Crystalline structure: whether the atoms or molecules exhibit the 'long-range order' evidenced in crystalline solids.With regard to the scattering of light, the most critical factor is the length scale of any or all of these structural features relative to the wavelength of the light being scattered. Nitrogen and oxygen are not greenhouse gases because there is no molecular dipole moment. At the atomic or molecular level, physical absorption in the infrared portion of the spectrum depends on the frequencies of atomic or molecular vibrations or chemical bonds, and on selection rules. ![]() For example, in most glasses, electrons have no available energy levels above them in range of that associated with visible light, or if they do, they violate selection rules, meaning there is no appreciable absorption in pure (undoped) glasses, making them ideal transparent materials for windows in buildings.
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