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[[File:Dichroic filters.jpg|thumb|right|[[Dichroic filter]]s are created using optically transparent materials.]]
In the field of [[optics]], '''transparency''' (also called '''pellucidity''' or '''diaphaneity''') is the [[physical property]] of allowing [[light]] to pass through the material without appreciable [[light scattering by particles|scattering of light]]. On a [[macroscopic scale]] (one in which the dimensions are much larger than the wavelengths of the [[photon]]s in question), the photons can be said to follow [[Snell's law]]. '''Translucency''' (also called '''translucence''' or '''translucidity''') allows light to pass through
When light encounters a material, it can interact with it in several different ways. These interactions depend on the [[wavelength]] of the light and the nature of the material. Photons interact with an object by some combination of reflection, absorption and transmission.
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*Crystalline structure: whether the atoms or molecules exhibit the 'long-range order' evidenced in crystalline solids.
*Glassy structure:
*[[Microstructure]]: Scattering centers include internal surfaces such as grain boundaries, [[crystallographic defect]]s, and microscopic pores.
*Organic materials:
{{Main|Light scattering}}
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===Transparent ceramics===
Optical transparency in polycrystalline materials is limited by the amount of light
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 600/15 = 40 [[nanometer]]s) 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>
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*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 ultra violet 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:
===Infrared: Bond stretching===
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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
==Optical waveguides==
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===Mechanisms of attenuation===
{{See also|Light scattering}}
[[File:Silica core fiber minimum attenuation.jpg|thumb|Experimentally measured record low attenuation of silica core optical fiber. At
[[Attenuation]] in [[Optical fiber|fiber optics]], also known as transmission loss, is the reduction in intensity of the light beam (or signal) with respect to distance traveled through a transmission medium. Attenuation coefficients in fiber optics usually use units of dB/km through the medium due to the very high quality of transparency of modern optical transmission media. The medium is usually a fiber of silica glass that confines the incident light beam to the inside. Attenuation is an important factor limiting the transmission of a signal across large distances. In optical fibers, the main attenuation source is scattering from molecular level irregularities ([[Rayleigh scattering]])<ref>I. P. Kaminow, T. Li (2002), Optical fiber telecommunications IV, [https://books.google.com/books?id=GlxnCiQlNwEC&pg=PA223 Vol. 1, p. 223] {{webarchive|url=https://web.archive.org/web/20130527231335/http://books.google.com/books?id=GlxnCiQlNwEC&q&f=false&pg=PA223 |date=2013-05-27 }}</ref> due to structural disorder and compositional fluctuations of the [[Amorphous solid|glass structure]]. This same phenomenon is seen as one of the limiting factors in the transparency of infrared missile domes.<ref>{{cite journal|author1=Archibald, P.S. |author2=Bennett, H.E. |editor-first1=Stephen A. |editor-first2=Geoffery |editor-last1=Benton |editor-last2=Knight |name-list-style=amp |title=Scattering from infrared missile domes|bibcode=1978SPIE..133...71A|journal=Opt. Eng.|series=Optics in Missile Engineering |volume=17|page=647|year=1978|doi=10.1117/12.956078|s2cid=173179565 }}</ref> Further attenuation is caused by light absorbed by residual materials, such as metals or water ions, within the fiber core and inner cladding. Light leakage due to bending, splices, connectors, or other outside forces are other factors resulting in attenuation. At high optical powers, scattering can also be caused by nonlinear optical processes in the fiber.<ref>{{cite journal|author=Smith, R.G.|title=Optical power handling capacity of low loss optical fibers as determined by stimulated Raman and Brillouin scattering|journal=Appl. Opt.|volume=11|issue=11|pages=2489–94|year=1972|doi=10.1364/AO.11.002489|pmid=20119362|bibcode=1972ApOpt..11.2489S}}</ref>
==As camouflage==
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