Editing Transparency and translucency (section)

=== 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>