<Li> 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 . Nitrogen and oxygen are not greenhouse gases because there is no absorption because there is no molecular dipole moment . </Li> <P> 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 . Primary material considerations include: </P> <Ul> <Li> Crystalline structure: whether or not the atoms or molecules exhibit the' long - range order' evidenced in crystalline solids . </Li> <Li> Glassy structure: scattering centers include fluctuations in density or composition . </Li> <Li> Microstructure: scattering centers include internal surfaces such as grain boundaries, crystallographic defects and microscopic pores . </Li> <Li> Organic materials: scattering centers include fiber and cell structures and boundaries . </Li> </Ul> <Li> Crystalline structure: whether or not the atoms or molecules exhibit the' long - range order' evidenced in crystalline solids . </Li>

Classify materials on the basis of their appearance