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Ear
2019-Mar-08, 01:43 AM
Why does light go through glass?

Hornblower
2019-Mar-08, 01:54 AM
Why does light go through glass?

Perhaps we should turn that around and ask why many substances other than glass or water are opaque rather than transparent. If we look simplistically at atoms without going into the electromagnetic or quantum mechanical aspects of their constituent particles and of light, we see nuclei and electrons that are about 1/100,000 of an atom's diameter. We could naively expect minimal obstacles for photons from such sparse material. At this point I am not prepared to give a proper explanation. Let's hope some of our other participants can help.

Jens
2019-Mar-08, 02:05 AM
Why does light go through glass?

Just a tiny bit more detailed, but essentially, when photons travel through a substance they can interact with electrons (pushing the electrons into a higher energy state, which is absorption). They can also be refracted (like they change direction) or are reflected (going back in the direction they came from). So what happens to light basically depends on that. Usually in a gas the photons simply pass through while being very slightly refracted. And in a liquid, which like gas has no crystal structure, they usually get refracted but not so much reflected. And in a solid, because of the crystal structure, they tend to be absorbed and cannot pass through. Actually, glass is like a liquid, so it acts that way with light. You can think of glass as a liquid that is very viscous, so almost doesn't flow at all.

Hornblower
2019-Mar-08, 03:11 AM
Just a tiny bit more detailed, but essentially, when photons travel through a substance they can interact with electrons (pushing the electrons into a higher energy state, which is absorption). They can also be refracted (like they change direction) or are reflected (going back in the direction they came from). So what happens to light basically depends on that. Usually in a gas the photons simply pass through while being very slightly refracted. And in a liquid, which like gas has no crystal structure, they usually get refracted but not so much reflected. And in a solid, because of the crystal structure, they tend to be absorbed and cannot pass through. Actually, glass is like a liquid, so it acts that way with light. You can think of glass as a liquid that is very viscous, so almost doesn't flow at all.

Diamond is a crystalline solid and is transparent.

Jens
2019-Mar-08, 03:43 AM
Diamond is a crystalline solid and is transparent.

I tried to use the word "tend to" a lot to show that it's not a set rule, but rather a tendency because it depends on the structure of the crystal and how it interacts with light.

profloater
2019-Mar-08, 09:18 AM
There is a frequency clue too, UV light is absorbed by window glass while IR and visible pass. Indeed in fibre optics light can go for kilometers through glass, bouncing off the walls of the fibres unless you bend the fibres, when at a critical angle the light escapes. When we look at IR light it can pass through substances we cannot see visible light through. The frequency of the absorbing spectrum of the material defines which light frequencies are absorbed.

As to the detail i remember Feynmann talking about photons entering water, (the same is true of glass) some enter and some reflect, we observe that but how each photon decides whether to penetrate or reflect is mysterious.

Cougar
2019-Mar-08, 02:43 PM
Why does light go through glass?

In a sense, it doesn't. Many (most?) of the light's photons get "absorbed" by atoms in the glass and then re-emitted. (Well, as Jens mentions, it's a bit more complicated than that, but "absorption and re-emission" is a simple way to look at it.) This explains why light going through tinted glass comes out tinted. The light can also just be scattered. There's an excellent Richard Feynman presentation/book about this: QED, The Strange Theory of Light and Matter.

Grey
2019-Mar-08, 04:41 PM
I remember a cool discovery in a course in electricity and magnetism. This was a classical treatment (it was the course using Jackson (https://en.wikipedia.org/wiki/Classical_Electrodynamics_(book)), for those of you familiar with it). One of the problems we had to solve involved looking at electromagnetic radiation crossing an interface between two media, and it was possible to show that, when the second medium was a strong conductor, like a metal, that led to the waves being reflected, and when the material was a stron insulator, the waves would pass through instead. So the reason metals reflect light well is precisely because they are excellent conductors, and at least part of the reason that materials like glass or diamond are transparent is because they are electrical insulators. It was a cool realization.

Obviously, that's not the whole story (wood is a good insulator, why isn't it transparent?). There are questions of absorption bands and internal scattering, which can be affected by the low level structure, and a lot of this has to be understood with a fully quantum treatment. But it's at least part of the story, and an interesting part, I thought.

Jeff Root
2019-Mar-10, 08:17 AM
Many (most?) of the light's photons get "absorbed" by atoms in
the glass and then re-emitted.
It must be nearly all of them, or the glass would be cloudy.

Why are nearly all of the photons re-emitted in the same
direction that the absorbed photons were going?

-- Jeff, in Minneapolis

Grey
2019-Mar-10, 05:15 PM
It must be nearly all of them, or the glass would be cloudy.

Why are nearly all of the photons re-emitted in the same
direction that the absorbed photons were going?Because they don't just interact with one atom. What we see is a superposition of each photon potentially interacting with all of the atoms in the material. You could imagine each of the atoms in the first layer of the glass potentially absorbing and re-emitting the photon, so that each one acts as a new source; potentially in any direction, but you'll find that the waves will constructively interfere going forward, but destructively interfere going in any other direction. Then those waves interact with the next layer of atoms in the material, and so forth. We'll also see that the phase velocity decreases as a result of these (potential) interactions, which accounts for the slowing of light as it passes through. Essentially, you'll end up with something very like the Huygens classical picture of light waves refracting.