Editing Transparency and translucency (section)

== Absorption of light in solids ==
{{More citations needed section|date=April 2021}}
When light strikes an object, it usually has not just a single frequency (or wavelength) but many. Objects have a tendency to selectively absorb, reflect, or transmit light of certain frequencies. That is, one object might reflect green light while absorbing all other frequencies of visible light. Another object might selectively transmit blue light while absorbing all other frequencies of visible light. The manner in which visible light interacts with an object is dependent upon the frequency of the light, the nature of the atoms in the object, and often, the nature of the [[electron]]s in the [[atom]]s of the object.

Some materials allow much of the light that falls on them to be transmitted through the material without being reflected. Materials that allow the transmission of light waves through them are called optically transparent. Chemically pure (undoped) window glass and clean river or spring water are prime examples of this.

Materials that do not allow the transmission of any light wave frequencies are called [[opacity (optics)|opaque]]. Such substances may have a chemical composition which includes what are referred to as absorption centers. Most materials are composed of materials that are selective in their absorption of light frequencies. Thus they absorb only certain portions of the visible spectrum. The frequencies of the spectrum which are not absorbed are either reflected back or transmitted for our physical observation. In the visible portion of the spectrum, this is what gives rise to color.<ref name="bbbb">{{cite book|author1=Simmons, J.  |author2=Potter, K.S. |name-list-style=amp |title=Optical Materials|publisher=Academic Press|year=2000}}</ref><ref name="aaaa">{{cite book|author=Uhlmann, D.R.|title=Optical Properties of Glass|publisher=Amer. Ceram. Soc.|year=1991|display-authors=etal}}</ref>

Absorption centers are largely responsible for the appearance of specific wavelengths of visible light all around us. Moving from longer (0.7&nbsp;μm) to shorter (0.4&nbsp;μm) wavelengths: Red, orange, yellow, green, and blue (ROYGB) can all be identified by our senses in the appearance of color by the selective absorption of specific light wave frequencies (or wavelengths). Mechanisms of selective light wave absorption include:

*Electronic: Transitions in electron [[energy levels]] within the atom (e.g., [[pigments]]). These transitions are typically in the ultraviolet (UV) and/or visible portions of the spectrum.
*Vibrational: [[Resonance]] in atomic/molecular [[vibrational modes]]. These transitions are typically in the infrared portion of the spectrum.

=== 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]]).

=== 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>

=== Transparency in insulators ===
An object may be not transparent either because it reflects the incoming light or because it absorbs the incoming light. Almost all solids reflect a part and absorb a part of the incoming light.

When light falls onto a block of [[metal]], it encounters atoms that are tightly packed in a regular [[Lattice model (physics)|lattice]] and a "[[sea of electrons]]" moving randomly between the atoms.<ref name="X">{{cite book|author1=Mott, N.F.  |author2=Jones, H.  |name-list-style=amp |title=Theory of the Properties of Metals and Alloys |publisher=Clarendon Press, Oxford (1936) Dover Publications (1958)}}</ref> In metals, most of these are non-bonding electrons (or free electrons) as opposed to the bonding electrons typically found in covalently bonded or ionically bonded non-metallic (insulating) solids. In a metallic bond, any potential bonding electrons can easily be lost by the atoms in a crystalline structure. The effect of this delocalization is simply to exaggerate the effect of the "sea of electrons". As a result of these electrons, most of the incoming light in metals is reflected back, which is why we see a [[Reflection (physics)|shiny]] metal surface.

Most [[Insulator (electricity)|insulators]] (or [[dielectric]] materials) are held together by [[ionic bond]]s. Thus, these materials do not have free [[conduction electrons]], and the bonding electrons reflect only a small fraction of the incident wave. The remaining frequencies (or wavelengths) are free to propagate (or be transmitted). This class of materials includes all [[ceramic materials|ceramics]] and [[glass]]es.

If a dielectric material does not include light-absorbent additive molecules (pigments, dyes, colorants), it is usually transparent to the spectrum of visible light. Color centers (or dye molecules, or "[[dopant]]s") in a dielectric absorb a portion of the incoming light. The remaining frequencies (or wavelengths) are free to be reflected or transmitted. This is how colored glass is produced.

Most liquids and aqueous solutions are highly transparent. For example, water, cooking oil, rubbing alcohol, air, and natural gas are all clear. Absence of structural defects (voids, cracks, etc.) and molecular structure of most liquids are chiefly responsible for their excellent optical transmission. The ability of liquids to "heal" internal defects via viscous flow is one of the reasons why some fibrous materials (e.g., paper or fabric) increase their apparent transparency when wetted. The liquid fills up numerous voids making the material more structurally homogeneous.{{Citation needed|date=July 2013}}

Light scattering in an ideal defect-free [[crystalline]] (non-metallic) solid that provides ''no scattering centers'' for incoming light will be due primarily to any effects of anharmonicity within the ordered lattice. Light [[transmission coefficient#Optics|transmission]] will be highly [[direction (geometry)|directional]] due to the typical [[anisotropy]] of crystalline substances, which includes their [[symmetry group]] and [[Bravais lattice]]. For example, the seven different [[crystalline]] forms of [[quartz]] silica ([[silicon dioxide]], SiO<sub>2</sub>) are all clear, [[transparent materials]].<ref>{{cite journal|author=Griffin, A.|title=Brillouin Light Scattering from Crystals in the Hydrodynamic Region|doi=10.1103/RevModPhys.40.167|journal=Rev. Mod. Phys.|volume= 40|issue=1|page=167|year=1968|bibcode=1968RvMP...40..167G}}</ref>