Thread: Why can visible light pass among glass?

Why can visible light pass among glass? Why glass is is transparent to visible light?

I guess we are talking about something like soda-lime-silica glass and not some other bizare stuff like fluoride- or chalkogenide glasses or the like.
Simple explanation: the network of the SiO2 tetrahedrons (well, throw in some network modifiers like lime or baking soda etc) shows two distinc absorption edge near IR (above ca. 2 micrometer wavelength) and near UV (soemwhat below 280 nm wavelength). This is due to two different effects:
a.) The IR gets absorbed due to vibrational modes of the Si-O and Na-O or Ca-O bonds
b.) The UV gets absorbed due to absorption by the electrons in the glass-network

For clear glass there is just nor esonance for the visible part of the electromagnetic spectrum, thus it passes through pretty much unabsorbed (if you let aside the absorption of iron-oxide contaminants in window glass).

Such Sarcasm!

To simplify that, it has allot to do with way the molecules are arranged. In a glass crystalline structure the molecules are arranged in perfect rows and sections, allowing the light to evenly float in between the molecules and out of the glass.

About what twang said (or at least it sounds right to me), he is right; but to simplify it-basically, the glass that cancels out IR and UV is black to those wavelengths. As in, if you only saw in those two, the glass would be black-or if reflective properties are present, it would appear as a mirror. But since you only see in the visible spectrum, the resultant loss of those two are not noticeable. Its as if a material absorbed green and blue, but not red-you would only see the red, but if green and blue were not viewable in the first place, it would not make any difference that they were now not being absorbed to your eyes; though of course, we eliminate UV not because we don't want to see it (which we can't), but because it adds to the damage rate of the cells in our eyes.

Originally Posted by Cold Fusion

To simplify that, it has allot to do with way the molecules are arranged. In a glass crystalline structure the molecules are arranged in perfect rows and sections, allowing the light to evenly float in between the molecules and out of the glass.

AAAAAAARRRRRRRRRHGGGGGGGGHHHH!!!!!!!

Glass DOES NOT HAVE A CRYSTALLINE STRUCTURE! No way.......the vitreous structure is a prime example for disordered structure. This is why it's called a glass or rather a glass is defined as a solid without long-range order or to put it correctly: the distribution function of the atoms in glass is non-periodic and can take any value between 0 and 1. And you can make glass out of (almost) every material. From metals to halogens to any kind of plastic. What the laymen call "glass" is something of the composition 70% SiO2, 10-15% CaO and 10 - 15% Na2O (in oxide nomenclature at least) plus some other minor components. A crystal can be described by it's elementary unit repeating in certain preferred spatial directions.

Originally Posted by Cold Fusion

About what twang said (or at least it sounds right to me), he is right;

Damn right, and as a Master in Materials Science and Engineering specialized in glass and ceramics I'd better be.

I thought that glass was a liquid though...

a solid without long range order? makes sence. Now I'm wondering about the line between solids and liquids. Glass does flow down. I know that.

Originally Posted by Shaderwolf

I thought that glass was a liquid though...

a solid without long range order? makes sence. Now I'm wondering about the line between solids and liquids. Glass does flow down. I know that.

Nope. You have been told that! Glass does not flow at room temperature. By no means and under no circumstances does window glass flow at room temperature. PERIOD. No actually it's PERIOD

Click on the link below and take a look at the picture. This shows the viscosity-temperature dependence of some "normal" glasses

http://www.a-m.de/deutsch/lexikon/vi...glas-bild1.htm


Can you see that the viscosity-curve never continues down to 25°C?? This is for a reason as it makes no sense whatsoever to go above 10^15 Poise. Even at the working point i.e. the temperature at which glass can be processed in a reasonable fashion the viscosity is 10^4 Poise and this is roughly the consistency of cold honey. The softening point i.e. the temperature where a defined body of glass starts to sag under it's own weight is at 10^7,6 poise so that's more than 1000 times the viscosity of the working point. And at the strain point i.e the temperature at which internal stresses in the glass from glassworking are relieved within an hour is 10^14,5 Poise. In the above curve the blue curve roughly is the composition of window glass, the light blue curve is a lead-borate glass which really is a "soft" glass with very low working point. You can actually take a glass-rod at this viscosity and tap it on the table and you can't tell the difference between this and a glass rod at room temperature. It's already that rigid. And if I had to give you a number for the viscosity of glass at room temperature I'd say from the top of my head 10^30 to 10^35 Poise. And if you take a look how viscosity is defined (shear rate under certain load and geometry) then you will see that at room temperature the sagging of a window pane in a Gothic cathedral to be noticed is in the ballpark of welllll.....10 to 100 times the age of our universe.

If you wanna know more: Arun Varshneya and David Pye have written some interesting books about this topic.

Thank you for your attention.

I feel like such a schmuck for perpetuating the glass flowing myth. You must have to debunk this on a near daily basis.

Originally Posted by Twaaannnggg

Glass DOES NOT HAVE A CRYSTALLINE STRUCTURE!

Heh. A while ago my wife and I were in a store and the clerk started trying to sell us some "glass crystal" dishes. I had just finished taking a solid-state materials class...it was sooooo hard to refrain from pointing out that "glass crystal" was a contradiction.

Mmmmm, I was told that glass does have a highly ordered structure; if you seem so certain, I guess they were wrong. But if the glass molecules are unordered, why is it that light perfectly lines up form beginning to end when passed through glass? Wouldn't it distort the light?

Originally Posted by Scifor Refugee

..it was sooooo hard to refrain from pointing out that "glass crystal" was a contradiction.

Mhmmmmm, but the term "crystal" for high-lead glasses is a historical one and it's too hard to get rid of this. It was coined to point out that the high refractive index of this glass makes for a nice sparcle resembling some diamond or zirconia crystal, that's all.

Originally Posted by Cold fusion

But if the glass molecules are unordered, why is it that light perfectly lines up form beginning to end when passed through glass? Wouldn't it distort the light?

Yes, glass influences light. That's called the refractive index and the refractive index is basically the inverse of the speed of light in this material compared with c in a vacuum. So in a glass light moves slower than in let's say air or the vacuum of space.

Which is why some glass has a bluish/ greenish tint?

You said you have degree in material sciece; what are you doing with the degree?

Originally Posted by Cold Fusion

Which is why some glass has a bluish/ greenish tint?

This is most of the time Iron(III) contamination in the raw material. It's really hard to get out of the sand and not really worth eliminating alltogether when you wanna have "ordinary" glass. The iron absorbs all but the green part of the visible spectrum and only lets the green through. As the absorption is also a function of distance of the light traveling in the absorbing material according to Lambert-Beer you can see this greenish tint along the edges of window panes. But there are other dopants one can use for green glass, e.g. chromium.

Originally Posted by Cold Fusion

You said you have degree in material sciece; what are you doing with the degree?

It hangs at my office wall

Actually I am Quality Mngr. at a plant of this Dutch electronics company.....you know the one that dicks over people with a totally useless coffe maker just for one cup where you have to buy these insanely expensive coffe-pads.

So you're the guy who makes sure that the coffee maker conks out after three uses?

Originally Posted by serpicojr

So you're the guy who makes sure that the coffee maker conks out after three uses?

Not really, I'm in the BU Lighting. But the BU Consumer Lifestile really got it made with that thing. I was just at some cash-and-carry electronics market and I saw the "new" designs of this thingamajig selling for 109€ (in words 109) retail price YIKES!!!!!! And take a look at the pads Douwe Egberts is selling! One of them goes for something like 0.5€! MUHAHAHAHA....can you say: "License to print money?"

Lol + I can see you have had change of heart by looking at your Avatar.

I need to upload one...but I'm too lazy to do all the stuff needed to do so.