Electromagnetic energy is either emitted or absorbed when an atom or molecule changes from one characteristic energy level to another. Need relative high energy for this to occur. Atoms are bound together by elastic forces. Reflectance spectra of moist soils include absorption bands at 1.4 m and 1.9 m, attributable to vibrational frequencies of water molecules in the soil. These bands are broad due to the unordered arrangement of the water molecules in the soil.
Rotational processes involve molecules and occur with microwaves.
In theory, the reflectance spectrum can be computed from knowledge of molecular and atomic structure and quantum mechanics, but soils are such complex mixtures of materials that this has been impossible thus far. The macroscopic interactions further complicate this. Water and OH- have absorption bands at 1.4 m. Most colors are due to preferential absorptions. The principal color is derived from the light that has penetrated and which, after being reduced by absorbing effect of the medium, is reflected by the second and third surface.