What makes glass transparent?

watch a house being built? Carpenters first erect the basic skeleton of the structure using two-by-four studs. Then they nail sheathing, usually plywood, to the studs to make walls. Most walls include a window opening, which holds a sheet of glass situated within a frame. Windows make a home feel bright, warm and welcoming because they let light enter. But why should a glass window be any more transparent than the wood that surrounds it? After all, both materials are solid, and both keep out rain, snow and wind. Yet wood is opaque and blocks light completely, while glass is transparent and lets sunshine stream through unimpeded.

You may have heard some people -- even some science textbooks -- try to explain this by saying that wood is a true solid and that glass is a highly viscous liquid. They then go on to argue that the atoms in glass are spread farther apart and that these gaps let light squeeze through. They may even point to the windows of centuries-old houses, which often look wavy and unevenly thick, as evidence that the windows have "flowed" over the years like the slow crawl of molasses on a cold day.

In reality, glass isn't a liquid at all. It's a special kind of solid known as an amorphous solid. This is a state of matter in which the atoms and molecules are locked into place, but instead of forming neat, orderly crystals, they arrange themselves randomly. As a result, glasses are mechanically rigid like solids, yet have the disordered arrangement of molecules like liquids. Amorphous solids form when a solid substance is melted at high temperatures and then cooled rapidly -- a process known as quenching.

In many ways, glasses are like ceramics and have all of their properties: durability, strength and brittleness, high electrical and thermal resistance, and lack of chemical reactivity. Oxide glass, like the commercial glass you find in sheet and plate glass, containers and light bulbs, has another important property: It's transparent to a range of wavelengths known as visible light. To understand why, we must take a closer look at the atomic structure of glass and understand what happens when photons -- the smallest particles of light -- interact with that structure.

Electron to Photon: You Don't Excite Me

First, recall that electrons surround the nucleus of an atom, occupying different energy levels. To move from a lower to a higher energy level, an electron must gain energy. Oppositely, to move from a higher to a lower energy level, an electron must give up energy. In either case, the electron can only gain or release energy in discrete bundles.

Now let's consider a photon moving toward and interacting with a solid substance. One of three things can happen:

1. The substance absorbs the photon. This occurs when the photon gives up its energy to an electron located in the material. Armed with this extra energy, the electron is able to move to a higher energy level, while the photon disappears.
2. The substance reflects the photon. To do this, the photon gives up its energy to the material, but a photon of identical energy is emitted.
3. The substance allows the photon to pass through unchanged. Known as transmission, this happens because the photon doesn't interact with any electron and continues its journey until it interacts with another object.

Glass, of course, falls into this last category. Photons pass through the material because they don't have sufficient energy to excite a glass electron to a higher energy level. Physicists sometimes talk about this in terms of band theory, which says energy levels exist together in regions known as energy bands. In between these bands are regions, known as band gaps, where energy levels for electrons don't exist at all. Some materials have larger band gaps than others. Glass is one of those materials, which means its electrons require much more energy before they can skip from one energy band to another and back again. Photons of visible light -- light with wavelengths of 400 to 700 nanometers, corresponding to the colors violet, indigo, blue, green, yellow, orange and red -- simply don't have enough energy to cause this skipping. Consequently, photons of visible light travel through glass instead of being absorbed or reflected, making glass transparent.

At wavelengths smaller than visible light, photons begin to have enough energy to move glass electrons from one energy band to another. For example, ultraviolet light, which has a wavelength ranging from 10 to 400 nanometers, can't pass through most oxide glasses, such as the glass in a window pane. This makes a window, including the window in our hypothetical house under construction, as opaque to ultraviolet light as wood is to visible light.