Editing Transparency and translucency (section)

==  Introduction ==
{{More citations needed section|date=April 2021}}
With regard to the absorption of light, primary material considerations include:
* At the electronic level, absorption in the [[ultraviolet]] and visible (UV-Vis) portions of the spectrum depends on whether the [[Atomic orbital|electron orbitals]] are spaced (or "quantized") such that electrons can absorb a [[quantum]] of light (or [[photon]]) of a specific [[frequency]]. For example, in most glasses, electrons have no available energy levels above them in the range of that associated with visible light, or if they do, the transition to them would violate [[selection rules]], meaning there is no appreciable absorption in pure (undoped) glasses, making them ideal transparent materials for windows in buildings.
* At the atomic or molecular level, physical absorption in the infrared portion of the spectrum depends on the [[frequencies]] of atomic or [[molecular vibrations]] or [[chemical bonds]], and on [[selection rule]]s. Nitrogen and oxygen are not greenhouse gases because there is no [[molecular dipole moment]].
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With regard to the [[light scattering in liquids and solids|scattering of light]], the most critical factor is the length scale of any or all of these structural features relative to the wavelength of the light being scattered. Primary material considerations include:
* Crystalline structure: whether the atoms or molecules exhibit the 'long-range order' evidenced in crystalline solids.
* Glassy structure: Scattering centers include fluctuations in density or composition.
* [[Microstructure]]: Scattering centers include internal surfaces such as grain boundaries, [[crystallographic defect]]s, and microscopic pores.
* Organic materials: Scattering centers include fiber and cell structures and boundaries.

{{Main|Light scattering}}

[[File:Diffuse refl.gif|thumb|right|250px|General mechanism of '''diffuse reflection''']]

[[Diffuse reflection]] - Generally, when light strikes the surface of a (non-metallic and non-glassy) solid material, it bounces off in all directions due to multiple reflections by the microscopic irregularities ''inside'' the material (e.g., the [[grain boundaries]] of a [[polycrystalline]] material or the [[Cell (biology)|cell]] or [[fiber]] boundaries of an organic material), and by its surface, if it is rough. Diffuse reflection is typically characterized by omni-directional reflection angles. Most of the objects visible to the naked eye are identified via diffuse reflection. Another term commonly used for this type of reflection is "light scattering". Light scattering from the surfaces of objects is our primary mechanism of physical observation.<ref name="z">{{cite book|author=Kerker, M.|title=The Scattering of Light|publisher=Academic, New York|year=1969|author-link=Milton Kerker}}</ref><ref name="y">{{cite journal
|author=Mandelstam, L.I.|title=Light Scattering by Inhomogeneous Media
|journal=Zh. Russ. Fiz-Khim. Ova.
|volume=58 |page=381
|year=1926}}</ref>

Light scattering in liquids and solids depends on the wavelength of the light being scattered. Limits to spatial scales of visibility (using white light) therefore arise, depending on the frequency of the light wave and the physical [[dimension]] (or spatial scale) of the scattering center. Visible light has a wavelength scale on the order of 0.5&nbsp;[[micrometre|μm]]. Scattering centers (or particles) as small as 1&nbsp;μm have been observed directly in the light [[microscope]] (e.g., [[Brownian motion]]).<ref>{{cite book|author=van de Hulst, H.C.|title=Light scattering by small particles|location=New York|publisher= Dover|year= 1981|isbn=0-486-64228-3}}</ref><ref>{{cite book|author1=Bohren, C.F.  |author2=Huffmann, D.R. |name-list-style=amp |title=Absorption and scattering of light by small particles|location=New York|publisher= Wiley|year= 1983}}</ref>

=== Transparent ceramics ===
Optical transparency in polycrystalline materials is limited by the amount of light scattered by their microstructural features. Light scattering depends on the wavelength of the light. Limits to spatial scales of visibility (using white light) therefore arise, depending on the frequency of the light wave and the physical dimension of the scattering center. For example, since visible light has a wavelength scale on the order of a micrometre, scattering centers will have dimensions on a similar spatial scale. Primary scattering centers in polycrystalline materials include microstructural defects such as pores and grain boundaries. In addition to pores, most of the interfaces in a typical metal or ceramic object are in the form of [[grain boundary|grain boundaries]], which separate tiny regions of crystalline order. When the size of the scattering center (or grain boundary) is reduced below the size of the wavelength of the light being scattered, the scattering no longer occurs to any significant extent.

In the formation of polycrystalline materials (metals and ceramics) the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during formation (or pressing) of the object. Moreover, the size of the grain boundaries scales directly with particle size. Thus, a reduction of the original particle size well below the wavelength of visible light (about 1/15 of the light wavelength, or roughly 600&nbsp;nm&nbsp;/&nbsp;15 = 40&nbsp;[[nanometre|nm]]) eliminates much of the light scattering, resulting in a translucent or even transparent material.

Computer modeling of light transmission through translucent ceramic alumina has shown that microscopic pores trapped near grain boundaries act as primary scattering centers. The volume fraction of porosity had to be reduced below 1% for high-quality optical transmission (99.99 percent of theoretical density). This goal has been readily accomplished and amply demonstrated in laboratories and research facilities worldwide using the emerging chemical processing methods encompassed by the methods of [[sol-gel]] chemistry and [[nanotechnology]].<ref>{{cite journal|author=Yamashita, I.|title=Transparent Ceramics|journal=J. Am. Ceram. Soc.|volume=91|issue=3|page=813|year=2008|doi=10.1111/j.1551-2916.2007.02202.x|display-authors=etal}}</ref>

[[Image:Backlit mushroom.jpg|thumb|Translucency of a material being used to highlight the structure of a mushroom]]
[[Transparent ceramic]]s have created interest in their applications for high energy lasers, transparent armor windows, nose cones for heat seeking missiles, radiation detectors for non-destructive testing, high energy physics, space exploration, security and medical imaging applications. Large [[laser]] elements made from transparent ceramics can be produced at a relatively low cost. These components are free of internal [[Stress (mechanics)|stress]] or intrinsic [[birefringence]], and allow relatively large doping levels or optimized custom-designed doping profiles. This makes ceramic laser elements particularly important for high-energy lasers.

The development of transparent panel products will have other potential advanced applications including high strength, impact-resistant materials that can be used for domestic windows and skylights. Perhaps more important is that walls and other applications will have improved overall strength, especially for high-shear conditions found in high seismic and wind exposures. If the expected improvements in mechanical properties bear out, the traditional limits seen on glazing areas in today's building codes could quickly become outdated if the window area actually contributes to the shear resistance of the wall.

Currently available infrared transparent materials typically exhibit a trade-off between optical performance, mechanical strength and price. For example, [[sapphire]] (crystalline [[alumina]]) is very strong, but it is expensive and lacks full transparency throughout the 3–5&nbsp;μm mid-infrared range. [[Yttria]] is fully transparent from 3–5&nbsp;μm, but lacks sufficient strength, hardness, and thermal shock resistance for high-performance aerospace applications. A combination of these two materials in the form of the [[yttrium aluminium garnet]] (YAG) is one of the top performers in the field.{{citation needed|date=August 2022}}