question: How is it that glass is rigid enough to hold its own shape, yet allows light to pass through?
comment: Also what part of the spectrum? Fiberglass in transparent in the microwave spectrum. The black die in the plastic cover of your tv remote is transparent in the IR yet black in the visible. Most optically clear glasses go opaque in the UV...
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.
   In order to reduce the amount of damaging light radiation transmitted through glazing, some glass coatings are designed to either "reflect" or "absorb" the ultraviolet (UV) spectrum. The following technologies are used to reduce the amount of UV from reaching the artwork:
   BULLET::::- The 1993 film "Philadelphia" features a scene in which a large conference room in the middle of the law firm has walls of glass on three sides. Jason Robards says, "Bill, will you get the windows?", and a switch is thrown, and all the windows immediately become translucent, so that no one can see them firing Tom Hanks' character.
   Section::::Influence on the science of glass.
   Glass is produced in standard metric thicknesses of 2, 3, 4, 5, 6, 8, 10, 12, 15, 19 and 25 mm, with 10mm being the most popular sizing in the architectural industry. Molten glass floating on tin in a nitrogen/hydrogen atmosphere will spread out to a thickness of about 6 mm and stop due to surface tension. Thinner glass is made by stretching the glass while it floats on the tin and cools. Similarly, thicker glass is pushed back and not permitted to expand as it cools on the tin.
  
  Section::::Prism glass.
   Section::::History.:Window materials.
   BULLET::::- UV Filtering of the Substrate is possible by adding UV filtering agents during the production of the substrate. While typical clear float glass blocks ~45% of UV radiation, the addition of CeOx to glass has been shown to further reduce UV transmission as well as widespread use of organic UV blocking dyes in the production of acrylic substrates. Most soda-lime glass completely absorbs short wavelength UV-B radiation below 300 nm. Low-iron glass typically blocks ~12% of UV radiation between 300 and 380 nm.
  
  Section::::UV Protection in Art Glazing.
  
  Section::::UV Protection in Art Glazing.:UV Definition in Art Framing.
   BULLET::::- At the electronic level, absorption in the ultraviolet and visible (UV-Vis) portions of the spectrum depends on whether the electron orbitals are spaced (or "quantized") such that they can absorb a quantum of light (or photon) of a specific frequency, and does not violate selection rules. For example, in most glasses, electrons have no available energy levels above them in range of that associated with visible light, or if they do, they violate selection rules, meaning there is no appreciable absorption in pure (undoped) glasses, making them ideal transparent materials for windows in buildings.
   "The tradition is that a merchant ship laden with nitrum being moored at this place, the merchants were preparing their meal on the beach, and not having stones to prop up their pots, they used lumps of nitrum from the ship, which fused and mixed with the sands of the shore, and there flowed streams of a new translucent liquid, and thus was the origin of glass."
   Section::::Structure.:Behavior of antique glass.
   Ninety percent of the world's flat glass is produced by the float glass process invented in the 1950s by Sir Alastair Pilkington of Pilkington Glass, in which molten glass is poured onto one end of a molten tin bath. The glass floats on the tin, and levels out as it spreads along the bath, giving a smooth face to both sides. The glass cools and slowly solidifies as it travels over the molten tin and leaves the tin bath in a continuous ribbon. The glass is then annealed by cooling in an oven called a lehr. The finished product has near-perfect parallel surfaces.
   BULLET::::- Light Transmission - the lower the light reflection and light absorption, the higher the light transmission, and therefore, the visibility of the objects displayed behind the glazing.
   SPDs can be manually or automatically "tuned" to precisely control the amount of light, glare and heat passing through.
  
  Section::::Electrically switchable smart glass.:Electrochromic devices.
  
  Electrochromic devices change light transmission properties in response to voltage and thus allow control over the amount of light and heat passing through. In electrochromic windows, the electrochromic material changes its opacity. A burst of electricity is required for changing its opacity, but once the change has been effected, no electricity is needed for maintaining the particular shade which has been reached. 
   Section::::Background.
   Section::::Introduction.
   The side of the glass that has been in contact with the tin has a very small amount of the tin embedded in its surface. This quality makes that side of the glass easier to be coated in order to turn it into a mirror, however that side is also softer and easier to scratch.
   Section::::History by culture.
  
  Section::::History by culture.:Iran.
   Low-iron, or water white glass, is made using special iron free silica, and is generally only available in 2.0 mm thicknesses for picture framing applications. Because low iron glass light absorption can be as low as 0.5% vs. ~ 2% for clear glass, the light transmission will be significantly better vs. clear glass. Low iron glass has light transmission of ~91.5% and reflection of 8%.
  
  Section::::Types of Picture Framing Glass.:Laminated Glass.
   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.
   Although protection is a primary purpose of glazing, "displaying" an artwork is the primary purpose of framing it. Therefore, the least visible glazing best displays the artwork behind it. Visible light transmission is the primary measure of glass' "invisibility", since the viewer actually sees the light, reflected from the artwork. Light transmission of glass is especially important in art framing, since light passes through the glass twice – once to illuminate the artwork, and then again, reflected from the artwork, as colors - before reaching the viewer.
   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.
   Light transmission (for this article, the perceivable visible spectrum between 390 nm and 750 nm is considered) through glass is diminished either by light reflection or light absorption of the glazing material. The total light transferred through the glazing material (light transmission) is reduced by reflection and/or absorption. In art framing, light reflection causes glare, while light absorption also may cause the transmitted colors to be dulled or distorted. While type of the glass substrate will affect the light absorption of the glazing, the surface treatment can affect light scattering, light reflection and in some cases, light absorption. There are various glazing options to achieve this goal as explained in the following sections on Types of Picture Framing Glass.
   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.
   Section::::Uses.
   The glass used for this purpose is typically whiter in colour than the clear glasses used for other applications.
  
  This glass can be laminated or toughened depending on the depth of the pattern to produce a safety glass.
  
  Section::::Float glass.
  
raw_presuppositions: Glass lets all light through.
raw_corrections: Glass can be made to only let certain lights through and not others. 