# Why is glass transparent and solid?

1. Apr 22, 2007

Glass is a solid? It is made up of molecules which is similar to what many other solids are made of? Then why is it transparent? Is it that the color of the molecules in glass are transparent and those in other solids are not? For that matter what is the color of molecules(atoms) in general?

2. Apr 22, 2007

### interested_learner

3. Apr 22, 2007

### lpfr

That is a very bad site to recommend.
First, this phrase is a nonsense (being polite):
"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."

And, second, there are not any explanations about why glass is transparent.

Tomorrow I will try to give an explanation if no one did before.

4. Apr 22, 2007

### ZapperZ

Staff Emeritus
You may want to try our FAQ first in the General Physics forum.

Zz.

5. Apr 24, 2007

### lpfr

Let's begin with "Why a substance is not transparent?
A substance may be not transparent because it reflects the incoming light or because it absorbs the incoming light. Of course, almost all substances reflect a part and absorb a part of the incoming light.

Metals reflect most of the light because they have free electrons. These electrons are shaken by the electric field of the the light which is an electromagnetic wave. Shaken electrons emit two waves. One in the direction of the incoming wave that is seen as the reflected wave and one similar in the same direction as the incoming wave which, added with it, give a zero amplitude wave.

Dielectrics do not have free electrons. So the electrons bound to atoms are less shaken and reflect just a fraction of the incident wave. As the amplitude of the generated wave is less, there is a wave that enters the dielectric. We are talking about a transparent dielectric, as glass.

If you put colorant (dye) molecules in the dielectric, they will absorb the power of the entering wave. This power is transformed in potential energy that can transform in heat or even in other kind of light in some substances. The electrons in the dye absorb energy by climbing to higher energy states in the molecule.

A dielectric (even a black one) always reflects a part of the incoming light. If it doesn't contain light absorbent molecules (pigments, dyes, colorants), it is transparent.

6. Apr 25, 2007

### cesiumfrog

A different question: could you make the "best" mirror from a crystal of titanium dioxide?

Normally we might use silver or mercury to make mirrors. Like you say, the free electrons make these highly reflective. However, these metals also appear "greyish". (On comparison with copper, which is clearly red-brownish, I'm assuming/guessing that the greyness results not just from an averaging of reflections in different directions but is actually due to the metal also behaving as a weak "colourant", absorbing some fraction of the light.)

Now, there are various materials that appear far more "white". One example of this is the titanium oxide colloid that forms the base of most paints. I assume this whiteness demonstrates these substances reflect almost all visible light (absorbing a smaller fraction of energy, and behaving as a worse black-body, than what a "greyish coloured" metal does). I also assume these materials normally produce diffuse (rather than specular) reflections because of the irregular shape of the surface (the relatively large particles composing the colloid). So, if we could put down perfectly flat layers of such a white material (say, grow a crystal then inspect its "perfect" surface) shouldn't it be expected to produce a better reflection than any ordinary metallic mirror?

Last edited: Apr 25, 2007
7. Apr 26, 2007

### lpfr

Mercury is not used anymore out of price and toxicity reasons. Current mirrors use silver and less current, aluminium.

Ti02 is not white. In fact it is transparent. But all powders look white because they reflect the white light in all directions. This is the case for sugar and salt. Both are transparent and look white. I do not know for sure why Ti02 is better than other bases used for paints, as ZnS. I presume that it is due to its high refraction index (2.6-2.9) that makes that more light is reflected each time it enters a crystal.
I you put Ti2 powder in the back face of a glass, you will get a very white glass, but not a mirror. If you are rich and put a monocrystal, you will get something like a glass but for which the reflected image is stronger than that of glass.

8. Apr 26, 2007

### cesiumfrog

OK, sounds reasonable, but not all powders look white (eg. coal dust). In fact, I naively expect powders to have darker colours for the same reason that cavities appear as black-bodies: regardlessly of how weakly the material surface may absorb light, by greatly increasing the ratio of material surface area to observable solid angle, all light gets bounced around internally for long enough to be absorbed. I think powdered silver is a very dark grey. The white-powder examples you gave were of transparent materials, so is total internal reflection the important mechanism that makes powders appear white?

9. Apr 27, 2007

### lpfr

Of course a lot of powders are not white. As I told, if there are molecules that absorb light, the powder is not white.
But a lot of colored substances seem white or clearer when powdered. This is because of reflections on the surface of grains.
To do a "black body" people used horn shaped holes. Successive reflections send the light inside until it fades away. But this "black body" is no better than the first reflection. Of course this first one must be as grazing as possible and the reflection should be specular (mirror like).
I don't remember seeing silver dust. But I bet that it is not black, unless it has been darkened by the sulfur in the air. I do have seen aluminium powder, that doesn't darken in air, and it is white.
For transparent crystals powder, total reflection may play a role, but I do not think that it is fundamental. This is only what I guess. I do not have the certitude.

10. Apr 27, 2007

### pixel01

I agree that all the metals are opaque because they contain a lot of free electrons which absorb the incoming photons. Graphite is also opaque for the same reason.
Now I have another old question, the reflection of polished metal surfaces. Why not the electrons inside the metal structures do absorb light but reflect it? Could anyone explain this ?
Thanks

11. Apr 27, 2007

### lpfr

Electrons in a dye molecule absorb light because there are energy levels where they can climb and stay. They give back this energy radiating other wavelengths.

Free electrons in a conductor can be considered as balls in a box. Electric field "shakes" them, as I wrote. Said in a more correctly way: the electric field creates an alternating current in the surface of the metal. The frequency of this current is the same as that of the EM wave (light in this case). As in all real metals this current heats the metal and this corresponds to loses in the reflection. This is the reason why a metal does not reflects 100% of the incoming light.
An electron can not absorb light otherwise than increasing its energy. It is either as potential energy as in dyes, or as kinetic energy as in conductors.
If it is as kinetic energy, it accelerates and an accelerated electron radiates an EM wave as I told you before.

12. Apr 27, 2007

### pixel01

I agree with you that at higher temperature, the reflection of polished metals is reduced. But the key point : why electrons do not absorb light but reflect it, is not explained yet.
Why graphite does not reflect (or just a little bit) ?
G and metals are all opaque for they have a lot of free electrons.

13. Apr 27, 2007

### lpfr

Who said that? Moreover I don't think it is true.