question: How can something as dense as glass be nearly completely transparent?
comment: One way to think of it is as an extension of how objects have color. Light is made of photons that have very specific energies. Atoms are made of electrons orbiting a nucleus. In order for light to be absorbed a photo must strike an atom, **and** have exactly the right energy to cause the election to jump. This specific energy requirement is created because there are only specific layers (think steps like a stair case) that an electron can rest at. Just like trying to walk up stairs, if you don't raise your foot enough (or raise it to high) you don't successfully manage to climb the stairs. So a blue object had atoms that Anson all sorts of colors (energies) of light, except blue. That one type doesn't match any electron energy level, so it either passes through or gets bounce off. So we see the object as blue. Glass is a material that doesn't have electrons capable of absorbing most visible light types. It does however, block infrared and UV colors very well. This also applies to why radio, also a "color" of light, can pass through most solid materials. Most materials don't have the correct atomic structure to interact with it.
passages: Other IR transparent ceramic materials that have been investigated include doped forms of CuO, MnO and SiO.
  
  Section::::Missiles.
   Section::::Missiles.:Composites.
   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.
   Section::::Absorption of light in solids.:Infrared: Bond stretching.
   Section::::Applications of SIL.:Optical data storage.
  
  Because SIL provides high spatial resolution, the spot size of laser beam through the SIL can be smaller than diffraction limit in air, and the density of the associated optical data storage can be increased.
  
  Section::::Applications of SIL.:Photolithography.
  
  Similar to immersion lithography, the use of SIL can increase spatial resolution of projected photolithographic patterns, creating smaller components on wafers.
  
  Emission Microscopy 
  
  Offers advantages for semiconductor wafer emission microscopy which detects faint emissions of light (Photons) from electron-hole recombination under the influence of electrical stimulation 
   Section::::Background.
   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 nanometers) eliminates much of light scattering, resulting in a translucent or even transparent material.
   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.
   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.
   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.
   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.
   Section::::Introduction.:Crystals.
   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.
   Section::::Structure.
  
  As in other amorphous solids, the atomic structure of a glass lacks the long-range periodicity observed in crystalline solids. Due to chemical bonding characteristics, glasses do possess a high degree of short-range order with respect to local atomic polyhedra.
  
  Section::::Structure.:Formation from a supercooled liquid.
   Furthermore, results indicate that microscopic pores in sintered ceramic nanomaterials, mainly trapped at the junctions of microcrystalline grains, cause light to scatter and prevented true transparency. It has been observed that the total volume fraction of these nanoscale pores (both intergranular and intragranular porosity) must be less than 1% for high-quality optical transmission, i.e. the density has to be 99.99% of the theoretical crystalline density.
  
  Section::::Lasers.
  
  Section::::Lasers.:Nd:YAG.
   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.
   Currently available infrared transparent materials typically exhibit a trade-off between optical performance, mechanical strength and price. For example, sapphire (crystalline alumina) is very strong, but it is expensive and lacks full transparency throughout the 3–5 micrometer mid-infrared range. Yttria is fully transparent from 3–5 micrometers, but lacks sufficient strength, hardness, and thermal shock resistance for high-performance aerospace applications. Not surprisingly, a combination of these two materials in the form of the yttrium aluminium garnet (YAG) is one of the top performers in the field.
  
  Section::::Absorption of light in solids.
   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.
   Transparent crystalline ceramics are used to defeat advanced threats. Three major transparent candidates currently exist: aluminum oxynitride (AlON), magnesium aluminate spinel (spinel), and single crystal aluminum oxide (sapphire). 
  
  Section::::Armor.:Aluminium oxynitride spinel.
   Section::::Applications.:Optical coatings.
   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).
   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
   Windows glass can be coated to achieve low emissivity or “low-e”. Some windows use laminate polyester film where at least one layer has been metalized using a process called sputtering. Sputtering occurs when a metal, most often aluminum, is vaporized and the polyester film is passed through it. This process can be adjusted to control the amount of metal that ultimately coats the surface of the film.
   BULLET::::- "Inverse opals" or "Inverse Colloidal Crystals"-Spheres (such as polystyrene or silicon dioxide) can be allowed to deposit into a cubic close packed lattice suspended in a solvent. Then a hardener is introduced that makes a transparent solid out of the volume occupied by the solvent. The spheres are then dissolved with an acid such as Hydrochloric acid. The colloids can be either spherical or nonspherical. contains in excess of 750,000 polymer nanorods. Light focused on this beam splitter penetrates or is reflected, depending on polarization.
  
  Section::::Fabrication challenges.
  
  Higher-dimensional photonic crystal fabrication faces two major challenges:
   In the processing of high performance ceramic nanomaterials with superior opto-mechanical properties under adverse conditions, the size of the crystalline grains is determined largely by the size of the crystalline particles present in the raw material during the synthesis or formation of the object. Thus a reduction of the original particle size well below the wavelength of visible light (~ 0.5 µm or 500 nm) eliminates much of the light scattering, resulting in a translucent or even transparent material.
  
raw_presuppositions: High density will decrease the transparency of a material.
raw_corrections: Density is much less important than whether the material is capable of absorbing visible light.