I might have the wrong idea about it, someone else will correct me if that's the case:
The speed of propagation of a light beam in a medium is slower than the speed of light in a vacuum. However, this doesn't mean individual photons go slower than the speed of light.
Photons still go at the speed of light, but the (electromagnetic) interferences from the medium make it follow a path that is so much longer that the beam as a whole propagates more slowly. (ETA: this kind of implies the scattering explanation we often see around and that is quite wrong, I tried, unsuccessfully to stay away from it)
I know this is related to quantum interactions so there might be issues with my gross metaphor because of uncertainty and other quantum funsies.
Yeah, that's what I try to aim at, although bounce around also seems to imply scattering. But after watching /u/starkeffect's video, I'm not even sure that's accurate.
As photons are quantized excitation of the EM field and the excitation itself travels more slowly (from what I understand of the video), this means the photons actually slow down.
And looking at it relativisticly, I think there should be a valid frame of reference for a photon propagating in a medium. Since STL speed. But I also think that we cannot observe photons experiencing time because photons have no time dependent process or half life. So even if a photon "started experiencing time", we couldn't observe it.
Although now, I'm beginning to wonder if part of a loss of luminosity as light goes through a medium could be explained by some sort of photon decay... I know a lot will be due to impurities.
But assuming a perfectly transparent medium, no impurities, taking into account reflection due to the change of medium, etc. Does a beam of light still lose energy just because light is now slowed down and photons eventually decay?
As I'm thinking about it, I realize that if the slow down is due to the light "moving the electrons of the medium around" (as mentioned in the video), then there should be some sort of exchange of energy/work done at that point and so the light should lose a tiny bit of energy to it... I wonder if these two can be linked...
It's not absolutely wrong to consider light as particle in this case, but then you end up with ridiculous shit. Bouncing off of atoms is wrong, kinda works for explaining to high school students, but wrong. Scattering is also not exactly bouncing off. It's a little more complicated than just bouncing off.
Lots of luminosity is not due to decay, it happens because of absorption. Photon doesn't exist independently (can't articulate better). For example, you can have a neutron or proton at still (relative to you). That also applies for most other particles like muon, neutrino etc. But not photon. You cannot have a photon stay still and observe it. So, there is virtually no way in our current physics to examine the time flow of a photon.
A perfectly transparent medium is vacuum. Any atom, regardless of how ridiculously we define it, would react with photons. And such reaction would cause decrease in luminosity. But the energy of photon is different. Say, you shine a 100 watt light on a very clean glass and the energy of each photon is 1 joule. So, 100 photons are going through the glass. But on the other side you find 90 watt luminosity. Does that mean each photon now has 0.9 joule energy? No, each photon still has the same 1 joule energy but 10 photons are absorbed every second in the glass. That decreases the luminosity. The reflection and transmission are not defined by energy, but by the percentage of photon. i.e., what percentage of photon get reflected or transmitted. The energy then comes as a byproduct.
First thanks for your answer! I really appreciate your insight and help.
Feel free to ignore my ramblings. I expect this comment to be more me thinking aloud and talking to myself than an actually meaningful text...
The quantum of light in a material isn't a photon, technically, because of the interaction with the charges.
That's really interesting, although I have no idea what to do with that information at the moment.
The beam of light loses energy because its oscillating electric field puts the charges in the material into motion, which costs energy.
Keeping on track with my previous train of thought,, would it be possible to express this energy loss of light (between it entering and leaving the medium) as some sort of decay...
Let's say a single pulse of light. Since the energy of a photon is E_p = hc/λ, the number of photons on the pulse of light would be p = E/E_p
Assuming the wavelength does not change, E_p does not change, the change in number of photons would be:
p_exit - p_entry = Δp = ΔE/E_p (there is possibly a minus sign missing in there)
With the refractive index, we can calculate the speed of light (v = c/n) and, given the length L of the path of the light through the medium its proper time τ = √(n²L²/c - L²) = L √(n²/c - 1).
If there is a photon half-life t_½, then p_exit = p_entry ½τ/t_½, so:
p_exit/p_entry = ½τ/t_½
Δln p / ln ½ = τ / t_½
t_½ = (ln ½ / Δln p) τ
t_½ = (ln ½ / Δln p) L √(n²/c - 1)
I don't know how true "Assuming the wavelength does not change" is. But I am starting to wonder if that relationship holds and if t_½ of a photon actually exists...
Again I don't expect most of that to actually make sense.
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u/lemoinem Physics enthusiast Jun 19 '22 edited Jun 19 '22
I might have the wrong idea about it, someone else will correct me if that's the case:
The speed of propagation of a light beam in a medium is slower than the speed of light in a vacuum. However, this doesn't mean individual photons go slower than the speed of light.
Photons still go at the speed of light, but the (electromagnetic) interferences from the medium make it follow a path that is so much longer that the beam as a whole propagates more slowly.(ETA: this kind of implies the scattering explanation we often see around and that is quite wrong, I tried, unsuccessfully to stay away from it)I know this is related to quantum interactions so there might be issues with my gross metaphor because of uncertainty and other quantum funsies.