question: How do transparent materials/substances exist?
comment: Think of a wire mesh, like an insert on a window. Because it’s so thin light get’s through no problem. It’s the same with most materials, if it’s thin enough some light will get through. Let’s add some thickness by stacking up layers of the wire mesh. After a few layers it’s hard to see through and after a few more it blocks the light just like a wall. But what if you line them all up just right? Not all the light would get through, but any light coming in straight would go right through the holes in the grid, same as before, right? What changed? The layout of the materials. Glass and other crystals are extremely organized structures, repeating the same patterns on the tiniest of scales. Because all of the atoms are lined up the same way it will let things through under the right circumstances.
passages: 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.
   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.
   Section::::Absorption of light in solids.:Infrared: Bond stretching.
   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 chart). 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 atom, one of several things can and will occur:
  
  BULLET::::- A molecule absorbs the photon, some of the energy may be lost via luminescence, fluorescence and phosphorescence.
   Latex rubber, which is naturally translucent, or plastics can be made into clothing material of any level of transparency. Clear plastic is typically only found in over-garments, such as raincoats. The use of translucent latex rubber for clothing can also be found in fetish clothing. Some materials become transparent when wet or when extreme light is shone on it, such as by a flashbulb.
  
  Section::::18th and 19th centuries.
   Materials which do not allow the transmission of any light wave frequencies are called opaque. Such substances may have a chemical composition which includes what are referred to as absorption centers. Most materials are composed of materials which are selective in their absorption of light frequencies. Thus they absorb only certain portions of the visible spectrum. The frequencies of the spectrum which are not absorbed are either reflected back or transmitted for our physical observation. In the visible portion of the spectrum, this is what gives rise to color.
   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.
  
  Section::::Absorption of light in solids.: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.
   Materials which do not transmit light are called opaque. Many such substances have a chemical composition which includes what are referred to as absorption centers. Many substances are selective in their absorption of white light frequencies. They absorb certain portions of the visible spectrum while reflecting others. The frequencies of the spectrum which are not absorbed are either reflected or transmitted for our physical observation. This is what gives rise to color. The attenuation of light of all frequencies and wavelengths is due to the combined mechanisms of absorption and scattering.
   BULLET::::- A molecule absorbs the photon which results in reflection or scattering.
  
  BULLET::::- 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).
   BULLET::::- Objects that [[Transparency and translucency|transmit light]] are either "translucent" (scattering the transmitted light) or "transparent" (not scattering the transmitted light). If they also absorb (or reflect) light of various wavelengths differentially, they appear tinted with a color determined by the nature of that absorption (or that reflectance).
   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.
  
  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.
   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.
   Section::::Introduction.:Crystals.
   Section::::Optical waveguides.:Mechanisms of attenuation.
   Other IR transparent ceramic materials that have been investigated include doped forms of CuO, MnO and SiO.
  
  Section::::Missiles.
   BULLET::::- Vibrational: Resonance in atomic/molecular vibrational modes. These transitions are typically in the infrared portion of the spectrum.
  
  Section::::Absorption of light in solids.: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 which compose the substance. In this case, the electrons will absorb the energy of the light wave and increase their energy state, often moving outward from the nucleus of the atom into an outer shell or orbital.
   Perceptual transparency
  
  Perceptual transparency is the phenomenon of seeing one surface behind another.
  
  In our everyday life, we often experience the view of objects through transparent surfaces.
  
  Physically transparent surfaces allow the transmission of a certain amount of light rays
  
  through them. Sometimes nearly the totality of rays is transmitted across the surface without
  
  significant changes of direction or chromaticity, as in the case of air; sometimes only light at
  
  a certain wavelength is transmitted, as for coloured glass.
  
  Perceptually, the problem of transparency is much more challenging: both the light rays
   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 a half a micrometer (one millionth of a meter). Scattering centers (or particles) as small as one micrometer have been observed directly in the light microscope (e.g., Brownian motion).
  
  Section::::Introduction.:Transparent ceramics.
   The first production of thin-film coatings occurred quite by accident. In 1817, Joseph Fraunhofer discovered that, by tarnishing glass with nitric acid, he could reduce the reflections on the surface. In 1819, after watching a layer of alcohol evaporate from a sheet of glass, Fraunhofer noted that colors appeared just before the liquid evaporated completely, deducing that any thin film of transparent material will produce colors.
   Section::::Missiles.:Composites.
   Light scattering in an ideal defect-free crystalline (non-metallic) solid which provides "no scattering centers" for incoming light will be due primarily to any effects of anharmonicity within the ordered lattice. Light transmission will be highly 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) are all clear, transparent materials.
  
  Section::::Optical waveguides.
   Photonics
  
  Photonics is the physical science of light (photon) generation, detection, and manipulation through emission, transmission, modulation, signal processing, switching, amplification, and sensing. Though covering all light's technical applications over the whole spectrum, most photonic applications are in the range of visible and near-infrared light. The term photonics developed as an outgrowth of the first practical semiconductor light emitters invented in the early 1960s and optical fibers developed in the 1970s.
  
  Section::::History.
   BULLET::::- 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 rules. Nitrogen and oxygen are not greenhouse gases because there is no molecular dipole moment.
  
  With regard to the 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:
  
  BULLET::::- Crystalline structure: whether or not the atoms or molecules exhibit the 'long-range order' evidenced in crystalline solids.
   Section::::Physical properties.:Optical.
  
  Materials can transmit (e.g. glass) or reflect (e.g. metals) visible light.
  
  Many materials will transmit some wavelengths while blocking others. For example, window glass is transparent to visible light, but much less so to most of the frequencies of ultraviolet light that cause sunburn. This property is used for frequency-selective optical filters, which can alter the color of incident light.
   Section::::Applications.:Optical coatings.
  