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Old 06-11-2019, 08:38 AM
KidCharlemagne is offline
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Question about light traveling through glass from an annoyed pre-dilletante.


Can I rant for a second about how frustrating it is to try get good information about the nature of light from popular books and the internet? How I can read/watch videos for hours about the double-slit experiment only to find out how popularizations of the concept make it seem so much "spookier" than it is by saying that it's almost as if the universe is making a conscious attempt to disguise the nature of light from experimenters and that I can only find the truth about it ten posts down in a question on the physics stack exchange?

Ok rant over.

So here's my latest: I thought I understood why light slowed down through glass - about how the energy of a photon was absorbed and re-emitted from multiple electrons and that the pathways of the daisy chain weren't linear, etc, etc. Then I come across a video on YouTube from some physics professor who certainly seemed credible who said he wanted to clear up misconceptions about light traveling through glass. He maintained that in order for a photon to be absorbed by an atom it's quanta of energy had to exactly match the energy required to move an electron to a higher energy state. Since the electrons in a glass lattice had particularly high energy jump requirements, photons could not be absorbed by the atoms and therefore kept moving on.

So which is it?

I understand that often the seeming contradictions in explanations on em waves are due to the inherent complexity of the subject, and that these explanations suffer from the "three blind men describing an elephant" effect, but the difference in these explanations seem to go beyond that.

As an aside, can anyone recommend a book on the full spectrum of electromagnetic waves - how they are created, interact, etc. that strikes a good balance between anthropomorphizing physical phenomena and including to many sigma symbols?
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Old 06-11-2019, 08:53 AM
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This might be a good start and has some useful links.
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Old 06-11-2019, 09:05 AM
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There's some wiggle room on how precise the energy needs to be in order to be absorbed. The further off from the exact energy you are, the less stable the resulting state will be. If you're not even close, then the re-emission will be almost instant.
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Old 06-11-2019, 09:57 AM
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There's a good discussion from FermiLab at https://www.youtube.com/watch?v=CUjt36SD3h8

In particular it spends some time discussing "false explanations on the internet" including the "photo constantly absorbed and re-emitted which slows it down" thing.
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Old 06-11-2019, 11:01 AM
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Originally Posted by KidCharlemagne View Post
So here's my latest: I thought I understood why light slowed down through glass - about how the energy of a photon was absorbed and re-emitted from multiple electrons and that the pathways of the daisy chain weren't linear, etc, etc. Then I come across a video on YouTube from some physics professor who certainly seemed credible who said he wanted to clear up misconceptions about light traveling through glass. He maintained that in order for a photon to be absorbed by an atom it's quanta of energy had to exactly match the energy required to move an electron to a higher energy state. Since the electrons in a glass lattice had particularly high energy jump requirements, photons could not be absorbed by the atoms and therefore kept moving on.

So which is it?
It definitely doesn't slow down due to absorption and re-emission, see the video leachim posted, it's pretty great.

And the bit about energy quanta is correct, but it's just part of the explanation, since glass is less transparent to long wave infrared, which is of lower energy.
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Old 06-11-2019, 11:54 AM
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As an aside, can anyone recommend a book on the full spectrum
ISWYDT.
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Originally Posted by KidCharlemagne View Post
of electromagnetic waves - how they are created, interact, etc. that strikes a good balance between anthropomorphizing physical phenomena and including to many sigma symbols?
It's certainly by no means a detailed description of all aspects of electromagnetic phenomena, but I highly recommend the short but emminently readable QED: The Strange Theory of Light and Matter by Feynman. It focuses specifically on the interactions between photons and electrons as described by quantum electrodynamics.
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Old 06-11-2019, 04:12 PM
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There's a good discussion from FermiLab at https://www.youtube.com/watch?v=CUjt36SD3h8

In particular it spends some time discussing "false explanations on the internet" including the "photo constantly absorbed and re-emitted which slows it down" thing.
I'm glad he appreciates all the bullshit explanations out there. And, mind you, it's not like I'm going to Cracked's Top Ten Things You Didnt Know About Light. I'm getting wrong info from physics teachers.

Moving on, why would the waves created by moving electrons be any slower than light? Are they not proper em waves?

Last edited by KidCharlemagne; 06-11-2019 at 04:12 PM.
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Old 06-11-2019, 04:19 PM
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ISWYDT.


It's certainly by no means a detailed description of all aspects of electromagnetic phenomena, but I highly recommend the short but emminently readable QED: The Strange Theory of Light and Matter by Feynman. It focuses specifically on the interactions between photons and electrons as described by quantum electrodynamics.
Funny that you mention him, because I was reading a paper by Art Hobson last night entitled "There Are No Particles, Only Fields," that seemed to be taking a swipe at him for holding on to the corpuscular theory of light. I've linked below. He quoted a colleague as saying that Feyman "attempted to remove field-particle dualism by getting rid of the fields.".

https://cpb-us-e1.wpmucdn.com/sites....-Particles.pdf
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Old 06-11-2019, 04:36 PM
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Of course the explanation about absorbtion and re-emission isn't really right. Any answer not in the language of quantum electrodynamics won't be really right. But it's about as good an approximation as you're going to get in layman's English.
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Old 06-11-2019, 07:41 PM
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I'd always understood that when light is absorbed and then re-emitted, the emission is in a random direction. That certainly is the case when it's gas molecules/atoms doing it. In fact, that's why the Greenhouse effect works. So if this happens in a solid, you'd expect that solid to be opaque.
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Old 06-11-2019, 08:09 PM
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Of course the explanation about absorbtion and re-emission isn't really right. Any answer not in the language of quantum electrodynamics won't be really right. But it's about as good an approximation as you're going to get in layman's English.
Not according to the guy in the Fermi lab video posted above. It doesn't appear to be a case of words failing either. Two different people have said they are different phenomena, and the guy mentions in the vido that light slowing through a transparent medium via absorptive/re-emission is a common misconception. He then goes on to give a reasonably layman's explanation of the truth.
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Old 06-11-2019, 08:10 PM
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I'd always understood that when light is absorbed and then re-emitted, the emission is in a random direction. That certainly is the case when it's gas molecules/atoms doing it. In fact, that's why the Greenhouse effect works. So if this happens in a solid, you'd expect that solid to be opaque.
Yes, that's one of the reasons cited in sources I've seen that the absorption/re-emission theory can't be true.
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Old 06-11-2019, 08:19 PM
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If the absorbtion were into a stable state, that would be true.
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Old 06-11-2019, 10:17 PM
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I have a more fundamental question: Why would an annoyed pre-dilletante want to be the one who sent light through glass?
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Old 06-11-2019, 11:58 PM
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If the absorbtion were into a stable state, that would be true.
IMO, the bigger problem with that explanation is that it's just not necessary. Photon absorption/emission is a quantum phenomenon. But you don't need QM to explain index of refraction. All you need is that light is a wave, and that materials have semi-free charges that move in response to these waves, and that these charges generate new waves but with a phase difference. It's entirely a classical explanation.

QM adds some corrections on top of this of course, but those are higher-order effects. The primary effect of a slower effective speed of light, but without scattering, has a completely satisfying explanation without resorting to nasty quantum stuff.
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Old 06-12-2019, 06:12 AM
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IMO, the bigger problem with that explanation is that it's just not necessary. Photon absorption/emission is a quantum phenomenon. But you don't need QM to explain index of refraction. All you need is that light is a wave, and that materials have semi-free charges that move in response to these waves, and that these charges generate new waves but with a phase difference. It's entirely a classical explanation.

QM adds some corrections on top of this of course, but those are higher-order effects. The primary effect of a slower effective speed of light, but without scattering, has a completely satisfying explanation without resorting to nasty quantum stuff.
Can you explain why the waves created by the electrons (or the net superpositioned wave) is effectively slower than the speed of light?
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Old 06-12-2019, 07:53 AM
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OK, then, the wave makes the electrons move, and the motion of the electrons produces a new wave, slightly delayed from the original. How is that any different than saying that the light is absorbed and re-emitted? So far as I can tell, the only difference is that it additionally specifies that the light is a wave, which is an unnecessary complication.
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Old 06-12-2019, 08:07 AM
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Sorry -- I didn't see this yesterday.


Have a look at this -- https://en.wikipedia.org/wiki/Ewald%...nction_theorem

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When light traveling in vacuum enters a transparent medium like glass, the light slows down, as described by the index of refraction. Although this fact is famous and familiar, it is actually quite strange and surprising when you think about it microscopically. After all, according to the superposition principle, the light in the glass is a superposition of:
The original light wave, and
The light waves emitted by each of the atoms in the glass.

(Remember, light has an electric field that pushes atoms back and forth, which causes the atoms to emit dipole radiation.)

Individually, each of these waves travels at the speed of light in vacuum, not at the (slower) speed of light in glass. Yet when the waves are added up, they surprisingly create only a wave that travels at the slower speed.

The Ewald–Oseen extinction theorem says that the light emitted by the atoms has a component traveling at the speed of light in vacuum, which exactly cancels out ("extinguishes") the original light wave. Additionally, the light emitted by the atoms has a component which looks like a wave traveling at the slower speed of light in glass. Altogether, the only wave in the glass is the slow wave, consistent with what we expect from basic optics.

A more complete description can be found in Classical Optics and its Applications, by Masud Mansuripur.[3] A proof of the classical theorem can be found in Principles of Optics, by Born and Wolf.[1], and that of its extension has been presented by Akhlesh Lakhtakia.[2]
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Old 06-12-2019, 10:24 AM
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Of course the explanation about absorbtion and re-emission isn't really right. Any answer not in the language of quantum electrodynamics won't be really right. But it's about as good an approximation as you're going to get in layman's English.
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If the absorbtion were into a stable state, that would be true.
The absorption, re-emission explanation is often given to and repeated by those of us who've never heard of absorption into a not-stable state, and without explaining how this negates the issues of the naive interpretation. Chiefly that the re-emission, at the level of knowledge I'm at, is in a random direction, and that's how we can look at emission spectra of a gas cloud.
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Old 06-12-2019, 10:46 AM
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Although if all photons were absorbed and re-emitted with a fixed time delay, then even if the re-emission occurred in random directions, they would constructively interfere only in the direction of the original direction, no?
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Old 06-12-2019, 11:14 AM
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Although if all photons were absorbed and re-emitted with a fixed time delay, then even if the re-emission occurred in random directions, they would constructively interfere only in the direction of the original direction, no?
I have a vague memory of my Interference and Diffraction professor writing out a bunch of ugly integrals showing something to this effect. *shudder*
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Old 06-12-2019, 03:51 PM
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Can you explain why the waves created by the electrons (or the net superpositioned wave) is effectively slower than the speed of light?
At a simple heuristic level, we can view electrons as a mass on a spring. The spring is their attraction to the nucleus, but they are also affected by passing waves.

Imagine that you have a weight hanging on a spring, and there is a second spring attached which you can use to pull the mass up and down. The mass won't follow your inputs exactly--the amplitude won't be the same, and the movement will be delayed compared to your inputs since the mass has inertia.

Light is the sum of all these waves. If you take a wave and add another wave with a phase shift, you'll get another phase shift. These tiny phase shifts, added up, lead to an effectively lower speed.

Note that the induced waves don't just go forward--they go backwards, too. The net effect is that all transparent materials are also at least partly reflective.
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Old 06-12-2019, 04:03 PM
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OK, then, the wave makes the electrons move, and the motion of the electrons produces a new wave, slightly delayed from the original. How is that any different than saying that the light is absorbed and re-emitted? So far as I can tell, the only difference is that it additionally specifies that the light is a wave, which is an unnecessary complication.
That light is treated as a wave is unavoidable. Index of refraction is dependent on wavelength... which requires a wave. Any theory that leaves that out is incomplete.

There's probably a way to make the absorption/emission approach work. I don't know; I haven't seen anyone try. But the classical approach where light is a wave moving through a bunch of charged oscillators explains most basic optical phenomena.

In the treatment I'm familiar with (from the Feynman Lectures), EM waves are never absorbed. They go on forever. The only way to block light is to set up some charges that oscillate exactly out of phase with the incoming light so that it deconstructively interferes past that point.

To some degree, this is just a philosophical point. Is there a difference between a "truly" null field vs. one that's the sum of exactly out-of-phase waves? Physically speaking, no--but there are times when making the distinction is helpful when working the math.
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Old 06-12-2019, 05:34 PM
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At a simple heuristic level, we can view electrons as a mass on a spring. The spring is their attraction to the nucleus, but they are also affected by passing waves.

Imagine that you have a weight hanging on a spring, and there is a second spring attached which you can use to pull the mass up and down. The mass won't follow your inputs exactly--the amplitude won't be the same, and the movement will be delayed compared to your inputs since the mass has inertia.

Light is the sum of all these waves. If you take a wave and add another wave with a phase shift, you'll get another phase shift. These tiny phase shifts, added up, lead to an effectively lower speed.

Note that the induced waves don't just go forward--they go backwards, too. The net effect is that all transparent materials are also at least partly reflective.
Perhaps we've gone as far as we can go with simple analogies, but I'd have thought the phase shifts would have just messed with the frequency, wavelength, and/or amplitude of the composite wave but keep the speed the same. I have a feeling I'm taking the wave analogy too literally at this point. So is this undergrad or master's levels physics? What would a class or textbook be titled that dealt with these concepts? Thanks a lot for your help!
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Old 06-12-2019, 05:55 PM
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So is this undergrad or master's levels physics? What would a class or textbook be titled that dealt with these concepts? Thanks a lot for your help!
Undergrad, but physics-major level. My usual source for things that go beyond the classes I took as an engineering undergrad are the Feynman Lectures, which are available online. In particular, we have The Origin of the Refractive Index for low-density materials, and then the Refractive Index of Dense Materials.

The math is not trivial. But the chapters are fairly readable even if you can't follow the math, so they're worth skimming at the least.

As for your particular concern, a harmonic oscillator driven by a wave will run at the same frequency as the driving wave, just at a different amplitude and maybe phase shift. That new wave, added to the original, results in another wave with the same frequency: adding any two waves with the same frequency, no matter what the phase and amplitude, gives a result wave with the same frequency.

Each interaction only has a small effect, but lots of tiny phase shifts added up result in a large phase shift. And a phase shift (delay) is exactly the same thing as saying the wave has slowed down. The light behaves just as if it had spent more time in the material as compared to a vacuum.
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