{"id":3349,"date":"2021-12-06T22:00:00","date_gmt":"2021-12-06T22:00:00","guid":{"rendered":"https:\/\/modernsciences.org\/staging\/4414\/?p=3349"},"modified":"2021-11-23T18:38:06","modified_gmt":"2021-11-23T18:38:06","slug":"why-are-glass-windows-transparent","status":"publish","type":"post","link":"https:\/\/modernsciences.org\/staging\/4414\/why-are-glass-windows-transparent\/","title":{"rendered":"Why Are Glass Windows Transparent?"},"content":{"rendered":"\n

Surely you\u2019ve 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\u2019s 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.<\/p>\n\n\n\n

At some point, some may have asked a pretty crucial question that\u2019s admittedly easy to miss: \u201cHow are these glass windows transparent, anyway?<\/em>\u201d It\u2019s a question that some have gone the entirety of their lives without ever thinking about, yet it\u2019s one of the most ubiquitous physical phenomena we see around us in our everyday lives. Let\u2019s take a deeper look.<\/p>\n\n\n\n

Let\u2019s Make Things Clear<\/h2>\n\n\n\n
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There is some interesting physics at play when you look out the window and see the clouds or the trees. (Todov, 2019) <\/figcaption><\/figure><\/div>\n\n\n\n

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.<\/p>\n\n\n\n

The puzzle piece that holds these two seemingly different things together is silicon dioxide (SiO2<\/sub>), otherwise known as silica<\/em>. Yes, it also comprises the stuff inside those silica gel<\/em> packets you see inside brand-new shoes or a new pack of rice crackers.<\/p>\n\n\n\n

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Beach sand is, for the most part, composed of silicon dioxide (SiO2<\/sub>) crystals. (Ktenopoulou, 2018) <\/figcaption><\/figure><\/div>\n\n\n\n

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.<\/p>\n\n\n\n

However, there\u2019s a way to stop this crystal growth, and it\u2019s the crucial step that differentiates between you making several million quartz crystals and you making glass: you quench<\/em> the molten silica\u2014usually by blasting it with pressurized air in various angles.<\/p>\n\n\n\n

This process rapidly cools the silica, preventing it from forming crystals in the process. As you\u2019ve effectively \u201cfrozen\u201d the silica molecules in place without giving them the chance to arrange themselves as a crystal, they\u2019re considered amorphous<\/em>, a term borrowed from what was ancient Greek for \u201cwithout form.\u201d Thus, in contrast to what would be otherwise a quartz crystal composed of silica molecules \u201cwith form,\u201d you instead made amorphous silica\u2014a glass<\/em>.<\/p>\n\n\n\n

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Silica molecules inside glass are frozen in place without being given the chance to form crystal structures; this means that glass is simply amorphous quartz. The phenomena that makes light simply pass through glass, however, is another story. (Zolotova, 2021) <\/figcaption><\/figure><\/div>\n\n\n\n

Now, that addresses the glass part of our problem\u2014which 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.<\/p>\n\n\n\n

Getting Into the Nitty-Gritty<\/h2>\n\n\n\n

The next thing to remember is that the photons<\/em> that carry light contain energy within them. If they didn\u2019t, then solar cell technologies simply wouldn\u2019t work. That analogy also awkwardly brings us to our next point, which is what happens when light interacts with a solid material.<\/p>\n\n\n\n

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<\/em> within the atoms that form the glass. (You need much<\/em> higher energies to convince the stuff within the atomic nucleus to move about\u2014that\u2019s the whole operating concept behind nuclear fusion<\/em><\/a>.)<\/p>\n\n\n\n

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Three things can generally happen when light strikes a solid: either the light gets reflected (left), absorbed (middle), or transmitted (right). (Maggie’s Science Connection) <\/figcaption><\/figure><\/div>\n\n\n\n

Now, these electrons that zip around atoms in a solid move through their energy levels<\/em>\u2014think how people move up and down layers of seating surrounding a concert venue\u2014by either absorbing or releasing energy at discrete amounts. These electrons can basically \u201csteal\u201d this energy from the photons of light that hit them, but only when the photon carries with it enough <\/em>energy for the electron to move. From there, three different things can happen.<\/p>\n\n\n\n

One, the energy from the photon of light can get absorbed<\/em> 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<\/em>. Alternatively, the target atom\u2019s electron can still absorb the energy from the incoming photon, but it also emits a photon with an identical <\/em>energy as a result; this is called reflection<\/em>.<\/p>\n\n\n\n

Third\u2014and perhaps most importantly for our discussion\u2014is the possibility that the energy that the photon carries isn\u2019t actually enough for the electron to move at all. Instead, the electron effectively \u201cignores\u201d the photon, and simply just lets it pass through. This is called transmission<\/em>\u2014and, as some of you might suspect, is the main reason why glass windows are see-through.<\/p>\n\n\n\n

Piece It All Together<\/h2>\n\n\n\n
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Ultimately, it\u2019s the way that incoming photons of light from the Sun interact with the electrons in the atoms within glass windows that make it transparent. (Do, 2019) <\/figcaption><\/figure><\/div>\n\n\n\n

So, let\u2019s all assemble it together. Glass windows are mostly made of amorphous silica, or silica molecules that didn\u2019t get enough time to arrange themselves into a proper crystal structure before solidifying.<\/p>\n\n\n\n

The energy carried by photons of visible light aren\u2019t 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<\/em> the glass when you look at it, instead of seeing the glass itself.<\/p>\n\n\n\n

One thing to note is that this phenomenon is restricted to photons of visible<\/em> light; photons of other wavelengths of light can have higher energies contained within them, and thus interact with electrons differently\u2014like those of ultraviolet <\/em>(UV) light. As a result, some glass windows are only partially<\/em> transparent to ultraviolet light; should you be able to see in UV light, glass windows would appear partially opaque.<\/p>\n\n\n\n

Similarly, this is also the reason why quartz crystals, also made of silica, are only partially<\/em> transparent. Compared to the roughly \u201cpure\u201d glass in windows, quartz crystals contain within them \u201cobstacles\u201d that would otherwise interact differently with incoming photons of light. These can include grain boundaries<\/em>\u2014a bit like a perimeter fence that separates crystalline grains <\/em>of different orientations within quartz\u2014and impurities<\/em> in the crystal, as is the case with tiny grains of silica sand on the nearest beach.<\/p>\n\n\n\n

And there you have it; now you know why glass windows are transparent. Now there\u2019s something new to ponder about. Maybe spend some time thinking about it\u2014perhaps by staring out a nearby window.<\/p>\n\n\n\n

References<\/h2>\n\n\n\n