Editing Transparency and translucency (section)

=== Infrared: bond stretching ===
[[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]]).

When a light wave of a given frequency strikes a material with particles having the same or (resonant) vibrational frequencies, those particles will absorb the energy of the light wave and transform it into thermal energy of vibrational motion. Since different atoms and molecules have different natural frequencies of vibration, they will selectively absorb different frequencies (or portions of the spectrum) of infrared light. Reflection and transmission of light waves occur because the frequencies of the light waves do not match the natural resonant frequencies of vibration of the objects. When infrared light of these frequencies strikes an object, the energy is reflected or transmitted.

If the object is transparent, then the light waves are passed on to neighboring atoms through the bulk of the material and re-emitted on the opposite side of the object. Such frequencies of light waves are said to be transmitted.<ref>{{cite book|author1=Gunzler, H.  |author2=Gremlich, H. |name-list-style=amp |title=IR Spectroscopy: An Introduction|publisher=Wiley|year= 2002}}</ref><ref>{{cite book|author=Stuart, B.|title=Infrared Spectroscopy: Fundamentals and Applications|publisher=Wiley|year=2004}}</ref>