About the interaction of light and solid matter

[snoopies622]

Last week I posted the following question here:

https://www.physicsforums.com/printthread.php?p=2735375

"Is it possible to predict the color of a solid object based on its molecular formula?"

It wasn't completely ignored but the thread did come to an abrupt end after only three responses, so I was wondering if I should have posted it in this section instead of the "general physics" section.

Basically I want to know why an object is whatever color it is. For example, to my left is a green box of tea. When white light hits it, green photons are reflected while other photons are absorbed. Why? I know that the electrons in an isolated atom have distinct energy levels and absorb only photons which can change their energy from one allowed level to another, but what about the case where there is a solid or liquid mix of atoms or molecules? Is the theory behind this established enough so that one say, "given a solid made of these particular molecules at this particular temperature, we know that it will be green"?

 

[alxm]

Well as I wrote there, these discrete levels smear out. http://brucegary.net/MTP_tutorial/OxyAbsSpec.png" , in that case of oxygen absorption at different altitudes, i.e. different pressures. As you see, at higher altitudes (low density) you have relatively distinct absorption peaks, and as you go towards low altitude/high density, it turns into one big continuum.

Yes, this can be predicted through quantum mechanics. It's not easy though, since for a solid you need to model it with a relatively big system, and for color, you would need a quite accurate method - since even a small error (in terms of energy) would be a rather large shift in color.

But colors can and have been predicted this way. But as of yet, it's not easier to calculate it than to measure it. Spectroscopy is very accurate.

 

[snoopies622]

Thanks, alxm. Could you give me a good source or two on this issue? I'm interested in the details.

 

[alxm]

Well, what do you want to learn?

The basic principles are from quantum mechanics. Then you have to work your way up to solid-state physics (for solids) or quantum chemistry (other phases) and so forth. It might be simplest to start reading up on UV-VIS-IR spectroscopy though, e.g. in physical chemistry textbooks.

Absorption/emission of visual light corresponds to either 1) The low end of electronic transitions. (Which are usually beyond the visual, in UV) or 2) The high end of vibrational transitions (which are usually down in the infrared).

For instance, metals don't absorb much visual light, which (together with their reflective property) gives them the silverish color we all know. But gold has an electronic transition which (compared to silver) is shifted down from the UV into the blue range. So it absorbs blue light and hence looks yellow.

Water is a very light blue. That is due to a vibrational mode, most of which is in the IR, but it also extends a bit into the red. But heavy water (D2O) isn't blue - because vibrational modes depend on the masses of the atoms (a spring with two heavy masses doesn't vibrate like a spring with two light ones). But electronic states don't depend on isotopes at all, really.

Anyway, the bottom line is that it's exceedingly difficult to predict colors, because doing so means you need to know all the different electronic and vibrational states, and you can only get this information by fully solving the Schrödinger Equation for the molecule or crystal in question, and to a high level of accuracy. (in fact, you can't even use the S.E. for gold, because the S.E. doesn't take special relativity into account, something which is important for heavy elements. In fact, the yellow color of gold is a relativistic effect, in the sense that it'd be silver-colored according to the S.E.)

 

[snoopies622]

Wow, thanks alxm. This is a good beginning for me.