Glass is so common that most of us take it completely for granted. But just what is it about glass that makes it transparent? Why can we see through a window and not through the wooden frame that surrounds it?

You have probably noticed that most liquids and gases are transparent. Water, cooking oil, rubbing alcohol, air, natural gas, etc. are all clear. That's because of a fundamental difference between solids, liquids and gases. When a substance is in its solid state, normally its molecules are highly organized in relation to one another, strengthening the bond between them and giving the substance rigidity. As the substance changes from a solid to a liquid, however, the strength of the bond lessens and the molecules begin to align themselves randomly. If we follow the substance's progression to a gas, we see that the molecular bond is greatly weakened and the relationship of the molecules to one another is almost completely random.

This progression from ordered to random organization is the primary reason that light can pass through liquids and gases. Just like bricks stacked neatly on top of one another, the ordered molecules of most solids are virtually impenetrable to light waves. Depending on the substance, the light waves will be reflected, scattered, absorbed or, more likely, some combination of the three. But as the substance changes to liquid or gas and the molecules are not stacked neatly anymore, gaps and holes occur that allow portions of the light waves to pass through. The greater the randomness of the molecular organization of the substance, the easier it is for the light to pass through.

Another factor happens at the sub-atomic level. The atoms that bind together to make the molecules of any particular substance have electrons, usually lots of them. When photons come in contact with these electrons, the following can occur:

  • An electron absorbs the energy of the photon and transforms it (usually into heat)
  • An electron absorbs the energy of the photon and stores it (this can result in luminescence, which is called fluorescence if the electron stores the energy for a short time and phosphorescence if it stores it for long time)
  • An electron absorbs the energy of the photon and sends it back out the way it came in (reflection)
  • An electron cannot absorb the energy of the photon, in which case the photon continues on its path (transmitted)

Most of the time, it is a combination of the above that happens to the light that hits an object. The electrons in different materials vary in the range of energy that they can absorb. A lot of glass, for example, blocks out 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. If the electrons absorb the energy of any portion of the visible spectrum, the light that transmits through will appeared colored according to the portion of the spectrum absorbed. In fact, the color of any object is a direct result of what levels of energy the electrons in the substance will absorb!

Although forms of glass, such as obsidian or volcanic glass, can occur naturally, Glass is generally a manmade substance. Here is the basic way to make glass:

  • Take the most common glass material, silica, which is just plain old sand like you would find on the beach.
  • Heat it to an extreme temperature until it becomes liquid, then cool it.
The resulting substance has a molecular structure that is very random like a liquid yet that retains the strong bond and rigidity of a solid. This is a simplification of the process. Usually you add both a substance to make the silica melt quicker and something else to stabilize it so that the glass is not brittle and easily broken. The temperature, heating time and cooling method must all be very exact.

The materials used for glass-making cool to form an amorphous mix of molecules (like a liquid) and have electrons that do not absorb the energy of photons in the visible spectrum. This is why you can see through glass, but not wood, metal or stone, which are all solids.

A similar method, called quenching, is used with plastics to make them transparent or translucent. Quenching causes the polymers (long-chain molecules) in the plastic to settle into a random pattern that allows light to pass through. You can even use this process with organic substances. Clear or translucent candy is created by heating the ingredients of the recipe and then rapidly cooling them.

Notice that clear glass, clear plastic and clear candy are all solids that are melted and then cooled. Same process!

Thousands of different substances are used to make various forms of glass. How much and what type of light is transmitted depends on the type and purity of the substance used. Silica, in its purest form, transmits light well. Very little of the light wave is absorbed, but some of it is usually reflected. Look at almost any window and you will see this is true.

Other materials used to make glass may transmit or block specific types of light, such as ultraviolet light, or even parts of the visible spectrum. You have probably seen glass that was black or some other opaque color. Most often the color is caused by microscopic particles suspended in the glass, like the impurities we talked about in some liquids and gases. Another way to change the properties of the glass, such as filtering specific wavelengths of light, is to slow down the cooling process enough to allow the molecules to partially crystallize, or form pattern. And finally, some materials are chosen because they can be shaped and made to transmit and/or refract light in specific ways to use, for instance, as eyeglass lenses or as a magnifying glass.

Here are several useful links: