A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

How come you can see through glass?

December 3, 1982

Dear Cecil:

OK, no bullshit now. I got a simple question, I want a simple answer: how come you can see through glass?

Dear Daniel:

Not to beat around the bush or anything, Dan, but the reason you can see through glass basically is that there is no reason for you not to be able to see through it. Despite its appearance, glass is really a highly viscous liquid rather than a solid, and you can see through it for the same reasons that you can see through water.

Having supplied that admirably simple answer, permit me to elaborate. Conventional liquids, when cooled, have a freezing point at which they suddenly become solid. Liquid glass, by contrast, simply gets gradually stiffer as it cools. At room temperature its rate of flow is so slow that it would take billions of years to ooze out of shape, and for most practical purposes it may be treated as a solid.

Its internal structure, though, is not the regular crystalline latticework of your standard solid, but rather is essentially random, like the typical liquid. As with many liquids, the rather loosely spaced molecules in glass are simply not big enough to obstruct the passage of light particles.

Furthermore, (a) there are no footloose electrons in glass to reflect light, as with metals; (b) the energy levels of the individual atoms in glass are not such that they absorb light in the visible spectrum, although they will absorb infrared and ultraviolet; and (c) there are no internal boundaries or discontinuities in glass as there are in ordinary crystal solids to refract light, which would cause some light to be lost to internal reflection. (Glass reflects light only at its external boundaries-- that is, the boundary between the glass and the surrounding air, or whatever. This permits refraction to be precisely controlled, which is what makes eyeglasses, and optics in general, possible.) In short, the reason you can see through glass is that there is no reason for you not to be able to see through it. QED.

BUSTED!

Dear Cecil:

WOW! I may have the distinct pleasure of catching the Straight Dope in an error. It regards a question that once enabled me to win a bet with a retarded ex-girlfriend and her Mensa mom: Is glass a liquid or a solid? I was taught that it was a supercooled liquid, and the dictionary concurred. What's more, in answering the question "How come you can see through glass?" you yourself said, "Despite its appearance, glass is really a highly viscous liquid rather than a solid." Needless to say, I was able to stick it in their proverbial eye. Recently, however, I have heard that glass isn't a liquid, it's an amorphous solid. Now who's going to open their eye big and wide for me? Please don't start tap-dancing and say it's all relative. We all know the world is black and white. Glass, solid or liquid? --Shayne Kislack

P.S.: Please let me know if I'm eligible for some sort of prize.

Cecil replies:

Now, Shayne. The mark of a truly great mind isn't whether you're right or wrong. It's how well you can weasel out of a jam.

Lesser folk might prefer it otherwise, but there's no sharp line dividing liquids and solids. A supercooled liquid, the term applied to glass for many years, has been rapidly chilled past its normal freezing point and become apparently solid without assuming the regular crystalline structure typical of solids. The term du jour, amorphous solid, means an apparently solid substance that lacks crystalline structure and instead has the random organization of liquids. In other words, we used to think of glass as a solidlike liquid, and now we think of it as a liquidlike solid. Big frickin' deal.

I concede that changes in the properties of glass once it cools past the "glass transition temperature" are an argument for calling it a solid. But to my mind the real question is whether glass flows, as liquids do. I'm happy to say it does, just not very fast. In the original column I wrote, "At room temperature [glass's] rate of flow is so slow that it would take billions of years to ooze out of shape." In the October 1999 issue of Discover, Yvonne Stokes, a mathematician at the University of Adelaide in Australia, says that it would take a mere ten million years for a windowpane to get 5 percent thicker at the bottom. So the way I see it, not only was I essentially right, I was being conservative by a margin of 100 to 1.

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