Surely you’ve gone and looked out the window at some point in your life; perhaps in a car, or on a bus, or just the one sitting between your bedroom or apartment and the outside world. And it’s a pretty nice-looking world out there, all things considered. No one would blame you for doing it, even if just to stare into the distance for a while.
At some point, some may have asked a pretty crucial question that’s admittedly easy to miss: “How are these glass windows transparent, anyway?” It’s a question that some have gone the entirety of their lives without ever thinking about, yet it’s one of the most ubiquitous physical phenomena we see around us in our everyday lives. Let’s take a deeper look.
Let’s Make Things Clear
Perhaps the first thing to note about glass is the fact that the base material that makes up your standard glass window is, chemically speaking, the same as the majority of the material that comprise the grains of sand on your nearest beach. Same likely goes for most glassware around your house, like wine glasses and punch bowls.
The puzzle piece that holds these two seemingly different things together is silicon dioxide (SiO2), otherwise known as silica. Yes, it also comprises the stuff inside those silica gel packets you see inside brand-new shoes or a new pack of rice crackers.
Now, to get glass out of silica, you melt down the silica into a hot goop. Letting it cool naturally and in an uninterrupted manner will cause its molecules to arrange themselves as a crystal. These crystals can number in the millions, as these crystals can form around almost any foreign presence in their vicinity: other silica crystals, specks of dust, bumps on the contact surface of whatever it is that contains the goop, and even air bubbles within the molten silica.
However, there’s a way to stop this crystal growth, and it’s the crucial step that differentiates between you making several million quartz crystals and you making glass: you quench the molten silica—usually by blasting it with pressurized air in various angles.
This process rapidly cools the silica, preventing it from forming crystals in the process. As you’ve effectively “frozen” the silica molecules in place without giving them the chance to arrange themselves as a crystal, they’re considered amorphous, a term borrowed from what was ancient Greek for “without form.” Thus, in contrast to what would be otherwise a quartz crystal composed of silica molecules “with form,” you instead made amorphous silica—a glass.
Now, that addresses the glass part of our problem—which brings us to the problem of what light, and in effect transparency, has to do with it. To answer this, we must first understand the way that light behaves when interacting with solids like glasses.
Getting Into the Nitty-Gritty
The next thing to remember is that the photons that carry light contain energy within them. If they didn’t, then solar cell technologies simply wouldn’t work. That analogy also awkwardly brings us to our next point, which is what happens when light interacts with a solid material.
To be specific, we need to talk about what happens when light strikes an atom. You see, the energy carried by photons of visible light is enough to jostle around only the electrons within the atoms that form the glass. (You need much higher energies to convince the stuff within the atomic nucleus to move about—that’s the whole operating concept behind nuclear fusion.)
Now, these electrons that zip around atoms in a solid move through their energy levels—think how people move up and down layers of seating surrounding a concert venue—by either absorbing or releasing energy at discrete amounts. These electrons can basically “steal” this energy from the photons of light that hit them, but only when the photon carries with it enough energy for the electron to move. From there, three different things can happen.
One, the energy from the photon of light can get absorbed by an electron that needs it, which causes the electron to jump to a higher energy level; the photon vanishes as a result, and this is called absorption. Alternatively, the target atom’s electron can still absorb the energy from the incoming photon, but it also emits a photon with an identical energy as a result; this is called reflection.
Third—and perhaps most importantly for our discussion—is the possibility that the energy that the photon carries isn’t actually enough for the electron to move at all. Instead, the electron effectively “ignores” the photon, and simply just lets it pass through. This is called transmission—and, as some of you might suspect, is the main reason why glass windows are see-through.
Piece It All Together
So, let’s all assemble it together. Glass windows are mostly made of amorphous silica, or silica molecules that didn’t get enough time to arrange themselves into a proper crystal structure before solidifying.
The energy carried by photons of visible light aren’t enough to be absorbed by electrons within silica molecules. As a result, the electrons in glass window atoms simply ignore the photons of visible light that hit them, letting them pass through in the process. This is why you mostly see the items behind the glass when you look at it, instead of seeing the glass itself.
One thing to note is that this phenomenon is restricted to photons of visible light; photons of other wavelengths of light can have higher energies contained within them, and thus interact with electrons differently—like those of ultraviolet (UV) light. As a result, some glass windows are only partially transparent to ultraviolet light; should you be able to see in UV light, glass windows would appear partially opaque.
Similarly, this is also the reason why quartz crystals, also made of silica, are only partially transparent. Compared to the roughly “pure” glass in windows, quartz crystals contain within them “obstacles” that would otherwise interact differently with incoming photons of light. These can include grain boundaries—a bit like a perimeter fence that separates crystalline grains of different orientations within quartz—and impurities in the crystal, as is the case with tiny grains of silica sand on the nearest beach.
And there you have it; now you know why glass windows are transparent. Now there’s something new to ponder about. Maybe spend some time thinking about it—perhaps by staring out a nearby window.
References
- Ford, M. (2001, January 22). How is tempered glass made? Scientific American. https://www.scientificamerican.com/article/how-is-tempered-glass-mad/
- Maggie’s Science Connection. (n.d.). Reflection, absorption & transmission. Maggie’s Science Connection. Retrieved 23 November 2021, from https://maggiesscienceconnection.weebly.com/reflection-absorption–transmission.html
- Tavel, M. (1999, October 21). What determines whether a substance is transparent? For instance, why is silicon transparent when it is glass but not when it is sand or a computer chip? Scientific American. https://www.scientificamerican.com/article/what-determines-whether-a/
- What makes glass transparent? (2000, June 19). HowStuffWorks. https://science.howstuffworks.com/question404.htm
