A not so original question about a not so original subject,

What is it about the atomic structure of glass that allows light to pass through it compared with,say, that of the metal Gold that is opaque and refuses to let light pass through it.

Or

Can an atom be described as transperent and if so how is the transparency achieved and yet "solidness" retained?

Noting that magnetic fields, gravity and Em fields are transparent.

How does an atom achieve the ability to reflect light?


etc etc

Any references would be welcome

The following is a SWAG explanation, which is a bit more reliable than a WAG explanation.

Matter is mostly empty space containing electromagnetic fields, atomic nuclei, and electrons. The nuclei and electrons occupy very small volumes compared to the overall volume.

One might wonder why most matter is opaque to light rays (more properly electromagnetic radiation).

I believe that light gets absorbed or reflected based on the energy (or frequency) of the incoming photons and the energy levels of the electrons in the material. For certain photon energies (or frequency relationships), no electromagnetic interaction takes place between the photons of light and the electrons (or electromagnetic fields) in the material. For other photon energies, the photons are absorbed or reflected. I think that in some cases, the light reflected is different from incoming light. For example: Visible light comes in and infrared ( heat) reflects. I think there are two types of out-going light: Reflection being one type; Absorption and re-radiation being the other.

Note that many materials opaque to visible light are transparent to X-rays and other types of light.

what on earth does SWAG and WAG mean?

A WAG is a Wild Ass Guess. A SWAG is a Sophisticated Wild Ass Guess.

This is technical jargon from the mainframe era. I guess the terms are archaic now. My spell checker approves of them, so they must still be known to some people.

A WAG is a Wild Ass Guess. A SWAG is a Sophisticated Wild Ass Guess.

An alternative meaning which was tought in nuke school was "Scientific Wild Ass Guess". Each has simular meaning. It is a guess or huntch based on some known facts or evidence.

ok then, ur question about transparency. hum, i kinda know and understand y, but i think im gona have to guess.

i would believe that the moleculare structure has some holes in it letting the light particules pass through it, reflect off of some object and come back to your eye.

NOTE: Glass gives off a glare indicating that it is not always transparent.

the reason, that i think, is y it gives off a glare is that the light molecules dont pass through the holes and end up boudcin off of the glass, reflecting off of something else, reflecting off of the glass again, and goin back to your eye. I can try to make a diagram, i just dont think i can post it.

I am not going to guess.

Certain materials are transparent because of their atomic structure. Electromagnetic radiation interacts with the electrons in the glass (or any other transparent material).

The electron can
* absorb the photon and transform its energy into thermal energy
* absorb the photon and keep the energy
* absorb the photon and quickly release the energy again (relfection)
* not absorb the energy of the photon and it keeps going

So only some frequencies of light will set up resonance with the electrons causing them to interact with the photon which will cause reflection, etc... But DO NOT say that photons do not interact with the electrons and pass straight through! OF course they are interacting. The thing is that it is the wavelength of the impinging photon which causes the transparency. It just so happens that glass molecules are transparent to photons with wavelengths around the 500nm length. Transparency of objects is a result of wavelength interactions with the electrons.

If we had evolved with Ultra-Violet sensitive eyes then glass would not appear transparent. Glass is only transparent to radiation with a wavelength that corresponds to the visible part of the electromagnetic spectrum.

* absorb the photon and quickly release the energy again (relfection)
I thought that reflection was a scattering process, not an absorption process?

Oxymoron: Contrary to the following, I strongly suspect that certain wavelengths pass through transparent media without interacting other than changing direction (refracting). . . . But DO NOT say that photons do not interact with the electrons and pass straight through! OF course they are interacting.I am not sure, but I think that the color or intensity might change if the photons interacted with the electrons in the atoms of the glass or other transparent medium. The change of direction might be indicative of some interaction, although refraction is related to the speed of light in the medium. Refraction might be an interaction with electromagnetic fields in between atoms and molecules, rather than interaction with the electrons in the atoms.

Perhaps we should stop making guesses here. Some where on the Web, there must be a valid description of the processes involved.

I am not guessing. That is how it is.

I regard refraction as an interaction. But the point was not about refraction. It was about explaining why some materials are transparent. Electromagnetic radiation does not just simply pass through material as it may have been interpreted in some of the previous posts. That is why I mentioned it.

I am not sure, but I think that the color or intensity might change if the photons interacted with the electrons in the atoms of the glass or other transparent medium. The change of direction might be indicative of some interaction, although refraction is related to the speed of light in the medium. Refraction might be an interaction with electromagnetic fields in between atoms and molecules, rather than interaction with the electrons in the atoms.

Of course. But this does not regard the question of transparency.

Perhaps we should stop making guesses here. Some where on the Web, there must be a valid description of the processes involved.

I'm not the one making guesses...

I am not sure, but I think

...hmmm

Oxymoron: You seem to know exactly what happens in a transparent medium. Could you explain more?

Do the incoming photons interact with photons or electrons in the interior of atoms, in the inter molecular spaces between atoms, or in the space between molecules? Does a transparent media absorb some photons? Perhaps those which happen to be going directly toward an atom or an atomic nucleus?

Refraction seems to be no more than a change of direction at the boundary between two media. As far as I know there is no change in frequency or intensity. What interaction takes place to cause the change of direction?

In some interactions, I believe that there is absorption of the energy of the incoming photons and re-radiation of new photons. I assume that neither refraction nor reflection involves absorption and radiation. Is this correct?

I thought that reflection was a scattering process, not an absorption process?

No, it is absorption. Incoming electromagnetic radiation interacts with the electrons on the surface (hence why most metals are shiny). The electrons absorb the energy and are promoted. Then they release the attained energy (obviously this is all quantized) in the form of light. Quantum Mechanics says that the electron will release exactly the same amount of energy that it absorbed (or none at all). That is why the amount of incoming light that is absorbed in this way is exactly reflected. Therefore, reflection is an absorption process.

Dinosaur, I will attempt to reply but I do not have my notes with me.

Do the incoming photons interact with photons or electrons in the interior of atoms, in the inter molecular spaces between atoms, or in the space between molecules? Does a transparent media absorb some photons? Perhaps those which happen to be going directly toward an atom or an atomic nucleus?

Lets take glass as our transparent medium. When photons enter the glass, all wavelengths of the electromagnetic spectrum are absorbed by the electrons. Meanwhile their visible counterparts are transmitted. Glass is of course a liquid so it is difficult for me to explain. However if we consider a metal (remember that some metals is transparent to UV), the photons with a UV wavelength are transmitted. The electrons that do the absorbing are resonating with the energy.

As you know, in metals the electrons are freely moving in the conduction band. From this we can assume that the electrons are free to resonate in this band and are being excited and de-excited. I will not bore you with the fundamentals of quantum excitation in photon-electron interactions.

The point here is that there is no energy loss in the process if Quantum Mechanics is to hold. Now if a photon's energy is almost equal to the fundamental energy state of the atom (metal) the more interaction there is. Since the energy of the emitted photon equals the energy of the absorbed photon the frequency does not change E = hf. However, due to absorption-emission there is a small time delay which ultimately increases the amount of time it takes for the photon to pass through the medium.

The slower average speed of light in the medium amounts to the medium having an index of refraction. And of course, the index of refraction implies refraction.

This is the same for any material, even glass. However, as I said before, glass is not a metal (or a solid for that matter). But the concept is the same. The photons have a time-delay from absorption-emission.

Refraction seems to be no more than a change of direction at the boundary between two media. As far as I know there is no change in frequency or intensity. What interaction takes place to cause the change of direction?

That is correct. There is no change in frequency. The change in direction can be seen directly by observing the well-known formula

Velocity = Wavelength * Frequency

Since frequency remains constant in refraction, and since we observe that velocity slows, it must be the wavelength which shortens to compensate.

We know have an explanation of why speed changes and wavelength changes. Now we adapt this to explain why the direction changes.

Every point wavefront can be thought of as a little wavelet (Huygen's Principle I think). This can be proven in the lab but for know we will just trust his perception. The wavefront hits a medium at an angle q, and the little wavlet which hits first has to obey the two points that I discussed earlier - slow and change wavelength. Therefore, if we follow in this manner along the wavefront, the wavefront looks like it is changing direction.

In some interactions, I believe that there is absorption of the energy of the incoming photons and re-radiation of new photons. I assume that neither refraction nor reflection involves absorption and radiation. Is this correct?

No. They both do. Ultimately, Quantum Mechanics explains any interaction by some sort of quantum process. In this case they are both explained by photon-electron interactions which (the only way they can) involves absorption-emission.