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Some materials, such as [[plate glass]] and clean [[water]], transmit much of the light that falls on them and reflect little of it; such materials are called optically transparent. Many liquids and aqueous solutions are highly transparent. Absence of structural defects (voids, cracks, etc.) and molecular structure of most liquids are mostly responsible for excellent optical transmission.
Materials
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.
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{{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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==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
Absorption centers are largely responsible for the appearance of specific wavelengths of visible light all around us. Moving from longer (0.7
*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.
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===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
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]].
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*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
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]]).
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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,
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>
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==Optical waveguides==
{{Main|Optical fiber}}
[[Image:Optical-fibre.svg|thumb|right|Propagation of light through a
[[Image:Laser in fibre.jpg|thumb|right|A laser beam bouncing down an [[Acrylic glass|acrylic]] rod, illustrating the total internal reflection of light in a multimode optical fiber]]
Optically transparent materials focus on the response of a material to incoming light waves of a range of wavelengths. Guided light wave transmission via frequency selective waveguides involves the emerging field of [[fiber optics]] and the ability of certain glassy compositions to act as a [[optical medium|transmission medium]] for a range of frequencies simultaneously ([[multi-mode optical fiber]]) with little or no [[Adjacent-channel interference|interference]] between competing wavelengths or frequencies. This resonant mode of energy and data transmission via electromagnetic (light) wave propagation is relatively lossless.{{Citation needed|date=April 2021}}
An optical fiber is a [[cylindrical]] dielectric waveguide that transmits light along its axis by the process of [[total internal reflection]]. The fiber consists of a [[core (optical fiber)|core]] surrounded by a [[Cladding (fiber optics)|cladding]] layer. To confine the optical signal in the core, the [[refractive index]] of the core must be greater than that of the cladding. The refractive index is the parameter reflecting the [[speed of light]] in a material. (Refractive index is the ratio of the speed of light in a vacuum to the speed of light in a given medium. The refractive index of vacuum is therefore 1.) The larger the refractive index, the more slowly light travels in that medium. Typical values for core and cladding of an optical fiber are 1.48 and 1.46, respectively.{{Citation needed|date=April 2021}}
When light traveling in a dense medium hits a boundary at a steep angle, the light will be completely reflected. This effect, called [[total internal reflection]], is used in optical fibers to confine light in the core. Light travels along the fiber bouncing back and forth off of the boundary. Because the light must strike the boundary with an angle greater than the [[total internal reflection|critical angle]], only light that enters the fiber within a certain range of angles will be propagated. This range of angles is called the [[acceptance cone]] of the fiber. The size of this acceptance cone is a function of the refractive index difference between the fiber's core and cladding. [[Optical]] [[waveguides]] are used as components in integrated optical circuits (e.g., combined with lasers or [[light-emitting diodes]], LEDs) or as the transmission medium in local and long
===Mechanisms of attenuation===
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