Editing Transparency and translucency (section)

=== UV-Vis: electronic transitions ===
In electronic absorption, the frequency of the incoming light wave is at or near the energy levels of the electrons within the atoms that compose the substance. In this case, the electrons will absorb the energy of the light wave and increase their energy state, often moving outward from the [[Atomic nucleus|nucleus]] of the atom into an outer shell or [[Atomic orbital|orbital]].

The atoms that bind together to make the molecules of any particular substance contain a number of electrons (given by the [[atomic number]] Z in the [[periodic table]]). Recall that all light waves are electromagnetic in origin. Thus they are affected strongly when coming into contact with [[negatively charged]] electrons in matter. When [[photons]] (individual packets of light energy) come in contact with the [[valence electrons]] of an atom, one of several things can and will occur:
* A molecule absorbs the photon, some of the energy may be lost via [[luminescence]], [[fluorescence]] and [[phosphorescence]].
* A molecule absorbs the photon, which results in reflection or scattering.
* A molecule cannot absorb the energy of the photon and the photon continues on its path. This results in transmission (provided no other absorption mechanisms are active).

Most of the time, it is a combination of the above that happens to the light that hits an object. The states in different materials vary in the range of energy that they can absorb. Most glasses, for example, block ultraviolet (UV) light. What happens is the electrons in the glass absorb the energy of the photons in the UV range while ignoring the weaker energy of photons in the visible light spectrum. But there are also existing special [[glass]] types, like special types of [[borosilicate glass]] or quartz that are UV-permeable and thus allow a high transmission of ultraviolet light.

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]]).