Transparency and translucency: Difference between revisions

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{{Short description|Property of an object or substance to transmit light with minimal scattering}}

{{Other uses ofotheruses|Transparency (disambiguation)|Translucence (disambiguation)}}

[[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''') allowsis the physical property of allowing light to pass through butthe doesmaterial not(with necessarilyor (again,without onscattering of light). It allows light to pass through but the macroscopiclight scale)does not necessarily follow Snell's law on the macroscopic scale; the photons canmay be scattered at either of the two interfaces, or internally, where there is a change in the index of [[refraction]]. In other words, a translucent material is made up of components with different indices of refraction. A transparent material is made up of components with a uniform index of refraction.<ref>{{cite journal |last=Thomas |first=S. M. |title=What determines whether a substance is transparent? |journal=[[Scientific American]] |date=October 21, 1999}}</ref> Transparent materials appear clear, with the overall appearance of one color, or any combination leading up to a brilliant [[spectrum]] of every color. The opposite property of translucency is [[Opacity (optics)|opacity]]. Other categories of visual appearance, related to the perception of regular or diffuse reflection and transmission of light, have been organized under the concept of [[Cesia (visual appearance)|cesia]] in an order system with three variables, including transparency, translucency and opacity among the involved aspects.

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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Transparency can provide almost perfect [[camouflage]] for animals <!--or possibly military equipment?--> able to achieve it. This is easier in dimly-lit or turbid [[sea]]water than in good illumination. Many [[marine biology|marine animals]] such as [[jellyfish]] are highly transparent.

[[File:Opacity Translucency Transparency.svg|thumb|250px|right|Comparisons of 1. opacity, 2. translucency with scattering, and 3. transparency; behind each panel (from top to bottom: grey, red, white) is a star.]]

== Etymology ==

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* 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]].

{{np}}{{pb}}

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.

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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: vondbond 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 10¹² [[cycles per second]] ([[Terahertz radiation#Natural|Terahertz radiation]]).

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[[Category:Physical properties]]

[[Category:Glass engineering and science]]

[[Category:Dimensionless numbers of physics]]