# The difference between transparent matter and opaque matter

Discussion in 'Chemistry' started by Magical Realist, Jan 6, 2013.

1. ### Magical RealistValued Senior Member

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Can anyone explain in layman's terms this difference? Is it a structural trait of assemblages of molecules, or a trait of the molecules themselves? Do certain molecules lend themselves to transparence more than others? Is the reason water is transparent the same as why glass is transparent? Just curious..

3. ### AlphaNumericFully ionizedModerator

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It is a bit of both. What matters is whether the material is capable of absorbing the energy of the photons, which means that the energy levels of the electrons in the system have a difference equal to the energy of the photon (a similar concept is discussed here).

For a single molecule you can use quantum mechanics (in principle) to work out the energy levels of the electron orbitals, $E_{n}$. A photon then comes into the molecule with energy $h\omega$ and so if there is a pair of orbitals where $E_{n}-E_{m} = h\omega$ then it can be absorbed. But it is made more complicated than that because for a molecule there can be energies in the bonds, which can flex and rotate, which can also carry energy and this can allow the molecule to absorb other energies too. It is made worse at higher temperatures, as these two types of energies smear together into a big complicated system but that isn't relevant here. This 'preference' for certain energies or energy ranges leads to phenomena like glass being transparent to visible light but opaque to infra red (can't use heat vision through windows).

For things like metals they do not form molecules in their pure state, they form a huge lattice of nuclei held in a configuration and the electrons move between them on much large, more complicated orbitals. However, now they can do this kind of free motion, rather than being restricted in the way they are in molecules, they can absorb pretty much any photon frequency (up to a point, x rays and gamma rays can just plough through a system sometimes).

The long and the short of it is that a substance is more likely to absorb a photon is there are two configurations whose energy difference is about the energy of the photon.

5. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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AlphaNumeric´s answer is correct, but one might add that the "valence electrons" (outer most electrons that determine the chemical properties) have quantized energy levels that are quite different in the isolated atom or molecule than when many are in a dense solid. In the dense solid the isolated atom´s (always read "and molecules" too) quantized level mutual interact (perturb each other) and the solid structure has bands of energy levels (many discrete levels so closely spaced that they form a band of levels).

Even one pure element, say carbon, does this, but the structure of the bands (their width and separation gaps) is strongly dependent upon the inter atomic spacing and crystal type. For example pure carbon as a diamond crystal is transparent (as I recall, for all the near IR, the visible and most of the near UV but when harsher UV photon try to pass thru it they have enough energy to boost and electron up into an available but unoccupied higher energy level and are absorbed.)

Indium is sort of the opposite case (if I recall correctly): A sheet of indium metal looks much like a sheet of lead - grey and not shiny. All the visible photons have adequate energy to boost electrons up to an available unoccupied energy level so indium absorbs light well, reflects light poorly and is opaque to the visible; however, indium does have a "band gap" between the valence electrons and the lowest available un occupied band levels. (I forget the width of this gap, but it is only a few tenths of an eV.) Thus when a near IR photon with less than this energy is incident upon indium sheet it passes right thru the sheet, like visible light passing thru glass. Indium can be and is used as an IR filter - blocking visible and letting you take photos of, for example the sun in the infra red, (if your detector is sensitive there - it probably needs to be cooled to liquid N2 temperature as if hotter it would blind itself with its own black body radiation.)

Aluminum is another interesting extreme: There is no band gap between the IR levels and the visible levels - just one wide energy band. The induced acceleration of the free electrons causes them to re-radiate IR thru visible photons, so Al reflects all these photons well as they all lack the energy to jump an election up into the available unoccupied higher band levels. A UV photon has the required energy, so it is absorbed, not reflected. If you plot the reflectivity of Al vs. wave length it is high for wave lengths longer than some UV wave length, then for the shorter photons, it rapidly falls very low.

Gold and copper get there color as they only reflect well the longer visible wave lengths. The shorter ones (like blue or green) have sufficient energy to be absorbed (jump an electron up into the higher available un occupied band). White light with little reflection of these shorter wave length is colored light.

All the above is because A+R+T = 1 always*, where A is the absorption coefficient, R the reflection coefficient and T is the transmission coefficient.

I don´t know for sure, but in addition to the dramatic changes in these coefficients that pure solid carbon has it switches structure I bet there is easily measured change in iron´s coefficients at a certain temperature when solid iron (Fe) changes between the body centered cubic and face centered cubic lattices structures.

Also only isotopic materials (random arrangements of atoms) like glass (which is really a “super cooled" liquid) have the same A,R,& T coefficients for all directions of photon travel, but this effect is typically small – Read about the shape of the Fermi surface and Brillion zones (may not be spelled correctly) in a solid state physic text.

* This is just the conservation of enery law, in disguise.

Last edited by a moderator: Jan 7, 2013

7. ### Magical RealistValued Senior Member

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Tks for both your enlightening posts. I remember back in one of the Star Trek movies they were talking about transparent aluminum. Is there any chance that would one day ever be invented?

8. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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No need to. If quite thin, it is transparent to the honey bee (I think) as they see far out into the UV - all the white flowers you see have strong colors for the bee.

Almost every thing is transparent if you chose the wave length correctly.

9. ### RhaedasValued Senior Member

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Trekkie nitpick: Although at the time it was just a name thrown out there to depict a very strong, light, and versatile material for starship construction, later Trek material as usual filled in some of the gaps. Particularly in the 4th movie, it's implied that transparent aluminum may be a type of improved plexiglass, or at least its inventor was involved in that technology.

There is something akin to transparent aluminum, an expensive ceramic. Aluminum Oxynitride, or AlON.

10. ### Magical RealistValued Senior Member

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Ahh..Thanks for the trekkie clarification.

11. ### Billy TUse Sugar Cane Alcohol car FuelValued Senior Member

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thanks. I have made transparent conductive films, cheaply and easily by spraying a tin containing solution (forget exactly what) onto hot substrates. Final product was called "stanous oxide." It is used still, I think, to make the widow airplane pilots look thru with electrical power de-icing of the window. This link is focused on applications for PV cells: http://en.wikipedia.org/wiki/Transparent_conducting_film which probably is the major market now.

I don´t know much about its mechanical properties, but on pilot´s window it obviously resists wind and high velocity water erosion well.