h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

How come you can see through several foot of glass but a cm of concrete is opaque and yet both are solid? I was asked this when someone found out that I'm a scientist but as a microbiologist it is way out of my field of expertise, but now I want to know. Darn curiosity.

An interesting one, to which I don't know the answer. What I would say though is that being solids might not have a lot to do with it. There are also liquids like milk which are reasonably opaque while water is quite see-through, and gases like chlorine which are more opaque than air.

Different substances interact with light in different ways. And it's those interactions in the visible region of the spectrum that affect the appearance as we see them.

Concrete is as transparent to neutrinos as is a block of glass.

If a substance does not absorb nor reflect visisble light then it will be transparent to it.
If it entirely reflects it then we have a mirror.

If it absorbs and relfects at different wavelenghts in the visible range it then it will be coloured according to which wavelengths it reflects back to your eye.

Traveller in Time probably corrected
"For what I understand of it it has to do with the molecules of the substance. Light waves have to be able to pass between the atoms so the wave length does also have its influence.

Milk is just a mixture of water and fat, the boundary between them acts as a mirror scattering the light in all directions.

Glass and many crystals are very homogene with relative large gaps between the atoms. Metals on the other hand have a close distance between the atoms and will be barely transparant. "

True enough Orcus. The question really is what makes them reflect (or not) light at the wavelengths they do?

Their chemical properties which are myriad.

Metals have a different electronic structure to most other substances, they have a broad 'sea' of electrons which will absorb light at pretty much all visible wavelenths and then spit it out again (i.e reflect it) and so they are good reflectors of light- giving the characteristic metal lustre.

Just to be clear, in 'clear' substances like quartz, diamond, glass etc. the bonding between atoms is by discrete chemical bonds that only absorb in the high-end UV range of the spectrum (quartz doesn't even absorb UV really). It is the interaction of light with electrons which gives substances colour.
It has nothing to do with the distances between atomic nuclei.

Yeah, nuclei spacing has nothing directly to do with the interaction of light with matter.


2 main types of light interaction with matter would be
1) reflection/scattering
2) absorption

Absorption fits under the description provided by Orcus about electronic levels in the material matching up with the energy associated with visible light. For example, red-coloured sports drink contains molecules that absorb blue light b/c the energy of a photon in blue light matches the energy level spacing in these molecules.

Reflection is easiest to describe from a metal surface. When an incoming beam of light strikes a metal surface, the free moving electrons of the metal re-arrange in response to the electric field of the light. The movement of these electrons produces an electric field, which has 2 components. One component travels into the metal itself, and completely cancels out the electric field of the incident light. The other component travels outward from the metal, and since it is at the same frequency as the incident light, it is light - and this is the reflected light we see.

If you have a rough surface, you may have reflection still being dominant, however the reflection will be randomly dispersed, and it won't look like the metal mirror case above (even if the physics are actually the same).

So when you see a solid object, it is solid through a combination of absorption and reflection.

I've read something about this very recently, a sort of Q&A thing, might have been in the Institute of Physics' newsletter... I'll dig it out.

Anyway, there the explanation was that transparency / non-transparency (absorbance) depends on the phonon spectrum of the material (in this context, phonon spectrum essentially means the power, at different light energies, that a material can absorb light energy into vibrations (ie, heat)). The argument was that, since glass is an amorphous, non-periodic solid, the phonon spectrum is poorly-defined, and, essentially, therefore glass lacks the ability to 'lock' light energy into these vibrations........ (That's still wavelength dependent, obviously)

The only problem with that is that the energy of phonon's is way to low to efficiently interact with visible light.

Interesting reading!
Just as a possible point of interest (although it has nothing to do with the properties of the material) here's a link to a concrete computer monitor:
http://www.innovationlab.net/sw22811.asp

Traveller in Time on looking in his optical fibre
"Interesting concept, however just beside the subject here. We all know light can travel through glass, embedding 'optical fibres' in a concrete slab does not make concrete transparent.

Notice the formulation it does not read glas fibers but fiber optics. "

(although it has nothing to do with the properties of the material)
". . . . as you warned us about. "

""The only problem with that is that the energy of phonon's is way to low to efficiently interact with visible light.""

... which is why glass is transparent to visible light. (But less so for things at lower energies, like infrared)

Sorry, that's not quite right...

However:

Phonon energies don't have to be equal to photon energies for there to an interaction. Otherwise, why does heating a light bulb filament cause it to emit visible light? One visible-energy photon cannot be absorbed by a material by creating One phonon, but it can by producing lots of them.

I don't know a lot about phonon physics. However, a quick glance through some literature abstracts shows phonon states in semiconductors with energies of ~ 0.04 eV. Visible light has energies on the order of an eV, so that's only a couple of orders of magnitude.

I do know a good deal about phonon, photon, and solid state physics.

Heating a light bulb causes electronic transitions to occur. You're eating that bulb to thousands of degrees C...Those electronic transitions produce the visible light observed.

Look up how LED's work, if you have access to the scientific literature/abstracts. You'll see they produce light by electronic transitions. An excited electron in a semiconductor relaxes back to the valence band, producing a photon of light.

There may sometimes be phonons coupled in with these processes (rarely), however I have never heard an experiment demonstrating the absorption of a visible light photon and the only result in the solid being creation of phonons.

I agree that phonon processes are unlikely to dominate most absorbtion processes. As I said in reference to the glass thing, I can only half remember the article. Why this guy thought that phonon processes were important in glass, I can't say. I'm only offering it as a discussion point.

You say "heating a light bulb filament causes electronic transitions to occur... Those electronic transitions produce the visible light observed".

Yes, there will be electronic transitions, and an electronic spectrum. But the test for incandescence is usually that you observe a black-body-like output spectrum. How can you get a black-body spectrum if it's just down to electronic transitions?

I beleive the blackbody radiation distribution merely indicates that effectively you have a continuum of available transitions. So, for example, when observing sodium in an arc lamp, this is not the case as you would only observe the discrete lines of the sodium atoms.

However, for a light bulb filament, the solid metal of the filament has a continuum of allowed electronic transistions (there is no band gap for the metal, there are no discrete levels of an atomic system).

Is 'see through' the same as 'transparent' and, if so, when did the word 'transparent' become unfashionable? It seems to have joined 'carcinogen' in the list of words newscasters won't use for fear of intimidating people who have no dictionary.