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Thus, when a material is illuminated, individual photons of light can make the [[valence electron]]s of an atom transition to a higher electronic [[energy level]]. The photon is destroyed in the process and the absorbed radiant energy is transformed to electric potential energy. Several things can happen, then, to the absorbed energy: It may be re-emitted by the electron as [[radiant energy]] (in this case, the overall effect is in fact a scattering of light), dissipated to the rest of the material (i.e., transformed into [[heat]]), or the electron can be freed from the atom (as in the [[photoelectric effect]]s and [[Compton scattering|Compton effects]]).
=== Infrared:
[[Image:1D normal modes (280 kB).gif|thumb|250px|Normal modes of vibration in a crystalline solid]]
The primary physical mechanism for storing mechanical energy of motion in condensed matter is through [[heat]], or [[thermal energy]]. Thermal energy manifests itself as energy of motion. Thus, heat is motion at the atomic and molecular levels. The primary mode of motion in [[crystalline]] substances is [[vibration]]. Any given atom will vibrate around some [[mean]] or average [[position (vector)|position]] within a crystalline structure, surrounded by its nearest neighbors. This vibration in two dimensions is equivalent to the [[oscillation]] of a clock's pendulum. It swings back and forth [[symmetrical]]ly about some mean or average (vertical) position. Atomic and molecular vibrational frequencies may average on the order of 10<sup>12</sup> [[cycles per second]] ([[Terahertz radiation#Natural|Terahertz radiation]]).
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