# Question about light traveling through glass from an annoyed pre-dilletante. **URL:** \ **Category:** Factual Questions\ **Created:** [June 11, 2019, 1:38pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374 "2019-06-11T13:38:51Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [June 11, 2019, 1:38pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/1 "2019-06-11T13:38:51Z")
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? ---
**Author:** ![bob\_2](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/bob_2/32/3341_2.png) [@bob\_2](https://boards.straightdope.com/u/bob_2)\ **Post date:** [June 11, 2019, 1:53pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/2 "2019-06-11T13:53:48Z")
[This](https://physics.info/light/) might be a good start and has some useful links. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 11, 2019, 2:05pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/3 "2019-06-11T14:05:01Z")
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. ---
**Author:** ![leahcim](https://avatars.discourse-cdn.com/v4/letter/l/b4bc9f/32.png) [@leahcim](https://boards.straightdope.com/u/leahcim)\ **Post date:** [June 11, 2019, 2:57pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/4 "2019-06-11T14:57:49Z")
There’s a good discussion from FermiLab at [Why does light slow down in water? - YouTube](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. ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [June 11, 2019, 4:01pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/5 "2019-06-11T16:01:35Z")
> [@KidCharlemagne](#): > > 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. ---
**Author:** ![markn\_1](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@markn\_1](https://boards.straightdope.com/u/markn_1)\ **Post date:** [June 11, 2019, 4:54pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/6 "2019-06-11T16:54:27Z")
> [@KidCharlemagne](#): > > As an aside, can anyone recommend a book on the full spectrum ISWYDT. > [@KidCharlemagne](#): > > 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. ---
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [June 11, 2019, 9:12pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/7 "2019-06-11T21:12:35Z")
> [@leahcim](#): > > There’s a good discussion from FermiLab at [https://www.youtube.com/watch?v=CUjt36SD3h8](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? ---
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [June 11, 2019, 9:19pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/8 "2019-06-11T21:19:58Z")
> [@markn\_1](#): > > 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.uark.edu/dist/b/383/files/2017/02/No-Particles.pdf](https://cpb-us-e1.wpmucdn.com/sites.uark.edu/dist/b/383/files/2017/02/No-Particles.pdf) ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 11, 2019, 9:36pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/9 "2019-06-11T21:36:25Z")
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. ---
**Author:** ![dtilque](https://avatars.discourse-cdn.com/v4/letter/d/d6d6ee/32.png) [@dtilque](https://boards.straightdope.com/u/dtilque)\ **Post date:** [June 12, 2019, 12:41am UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/10 "2019-06-12T00:41:39Z")
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. ---
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [June 12, 2019, 1:09am UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/11 "2019-06-12T01:09:09Z")
> [@Chronos](#): > > 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. ---
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [June 12, 2019, 1:10am UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/12 "2019-06-12T01:10:49Z")
> [@dtilque](#): > > 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. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 12, 2019, 1:19am UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/13 "2019-06-12T01:19:36Z")
If the absorbtion were into a stable state, that would be true. ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [June 12, 2019, 3:17am UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/14 "2019-06-12T03:17:03Z")
I have a more fundamental question: Why would an annoyed pre-dilletante want to be the one who sent light through glass? ---
**Author:** ![Dr.Strangelove](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dr.strangelove/32/6613_2.png) [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)\ **Post date:** [June 12, 2019, 4:58am UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/15 "2019-06-12T04:58:17Z")
> [@Chronos](#): > > 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. ---
**Author:** ![KidCharlemagne](https://avatars.discourse-cdn.com/v4/letter/k/ed8c4c/32.png) [@KidCharlemagne](https://boards.straightdope.com/u/KidCharlemagne)\ **Post date:** [June 12, 2019, 11:12am UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/16 "2019-06-12T11:12:32Z")
> [@Dr.Strangelove](#): > > 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? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [June 12, 2019, 12:53pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/17 "2019-06-12T12:53:04Z")
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. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [June 12, 2019, 1:07pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/18 "2019-06-12T13:07:18Z")
Sorry – I didn’t see this yesterday. Have a look at this – [Ewald–Oseen extinction theorem - Wikipedia](https://en.wikipedia.org/wiki/Ewald%E2%80%93Oseen_extinction_theorem) > [@](#): > > 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] ---
**Author:** ![naita](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/naita/32/5862_2.png) [@naita](https://boards.straightdope.com/u/naita)\ **Post date:** [June 12, 2019, 3:24pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/19 "2019-06-12T15:24:18Z")
> [@Chronos](#): > > 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. > [@Chronos](#): > > 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. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [June 12, 2019, 3:46pm UTC](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374/20 "2019-06-12T15:46:08Z")
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? [Next page](https://boards.straightdope.com/t/question-about-light-traveling-through-glass-from-an-annoyed-pre-dilletante/835374.md?page=2)