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jacobolus
hadlock
For those without a lot of patience, skip ahead to 6:45 or so (in the fist linked video) which is a really cool, "a-ha!" moment of why light "slows down" when it hits light. He also points out that he's really just animating feinman lectures, so giving credit where it's due. Gonna have to go dig out those books. Big thanks to jacobolus for linking these videos
Kranar
To be clear there are numerous explanations for why light slows down in a medium and they are almost all incorrect in one way or another.
This video uses the explanation that light interacts with electrons in a medium which causes the electrons to produce an electromagnetic wave whose interference pattern changes the phase of the light wave, kicking it back.
The problem is that an interference pattern can never change the speed of a wave, it can change the phase of the wave and adjust it, but that won't explain how light passing through a medium takes longer to traverse that medium than light travelling in a vacuum.
I don't know of a visualization or explanation that captures why light slows down in a medium, and the video linked is still a good way to get a sense of what's happening, but it's worth knowing that there is still a great deal being left out of that explanation.
It is nevertheless better than explanations involving the absorption an reemission of photons or that light bounces around within the internal structure of a medium. Both of those explanations manage to explain how light takes longer to pass through a medium but fails to explain how light keeps a consistent direction as opposed to scattering randomly.
The explanation in the video manages to explain how light maintains a consistent direction but fails to explain how light will actually take longer to pass through a medium compared to a vacuum.
Tazerenix
The actual answer is the assumptions which define a self-propagating wave do not apply once the wave leaves a vacuum. When it becomes incident onto some medium, due to the coupling of electrons within the medium to the electromagnetic field, the pure electromagnetic wave gets transformed into a phonon, which is a combination of electromagnetic and mechanical oscillation within the medium (and therefore has speed <c, depending on the particular properties of the medium). When the phonon subsequently leaves the system, those traveling oscillations induce a new self-propagating wave on the other side, sending the light on its way as usual.
gpsx
I'm not sure if this will be satisfying because it is a mathematical explanation rather than an intuitive one. If you combine two of maxwell's equations, you get a wave equation for electric/magnetic fields. However, there is an extra term in there, which is the effect of a time changing current on the magnetic field. In a material, the time changing electric field induces such a current, which in turn has the mathematical effect of reducing the coupling between the electric and magnetic field, creating the equation for a wave that is slower.
Something that might be comforting about light going slower than the speed of light is that the speed of light is not a special property of light, but it is a property of space time. It just so happens that light is the only way we have experience on this magic speed. Light isn't physically bound to go "the speed of light".
mikewarot
I thought that Grant's video did a great job of explaining it was all phase kicks, and how it all depended on resonant frequencies of the medium.
[Edit]In the second video above, he notes that phase kicks don't change the speed of the wave, even though the wavelengths can stretch out, it's still lower than the speed of light in vacuum.
dustingetz
visualized well in “Picture Book of Quantum Mechanics” ch 1 (a fairly hardcore physics text that does indeed have many diagrams)
martincmartin
No need to dig out the books, they're available online, well formatted and a pleasure to read:
SAI_Peregrinus
Huygens Optics (pretty much the only dedicated optics channel on Youtube) also recently made a video on this subject[1].He made some diffractive lenses via photolithography, and shows how the interference caused by variably spaced diffraction patterns causes refraction.
platz
One remaining question I had after watching Grant's video was that, although the collective behavior of a bunch of light waves made sense via interference, how would a single photon's direction would change upon entering the medium?
The best answer I could find on physics stack exchange was that the single photon's wavefunction is delocalized, so that the photon's wavefunction, in fact, interacts with the entire medium, instead of at a single point.
Is this the correct way to state this phenomenon or is there a better understanding of how the light emitted from a single photon interacting with a medium would bend?
dcreater
Grant has singlehandedly taught me more than all of my elite education combined. Man is an absolute treasure
luispauloml
AhnLab Safe Transaction's anti-phishing protection erroneously blocks this domain. The log says:
Date: <redacted>
Security Threat: Phishing site
Target: skullinthestars.com/
Status: Block
Scan Method: Anti-Phishing
For the curious ones, a related thread: "South Korea’s online security dead end" https://news.ycombinator.com/item?id=34231364
zppln
> We and our 772 advertising partners store and/or access information on your device [...]
> 772
Seriously...
lm28469
A lot sites are like that (most big ones), that's the whole reason they started GDPR. All you have to do is add a few widgets/plugin/third party ads/traffic monitors/tracking and their dependencies and you'll get hundreds of "partners"
arstechnica, cnet, &c. all have 200+ "partners"
xnorswap
And unfortunately they've all settled on the apparently untested, "legitimate interest" loophole where these hundreds of partners have a "legitimate interest" in serving you personalised adverts.
ycombinatrix
thanks for the heads up. i wasn't that interested in this topic anyway.
aurizon
There is an old science fiction concept of 'slow glass', where light travels very very slowly, with 1/2 day glass used for windows that allow light to dally for 1/2 day before it exits - also good for street lights. Extending it to week/glass, year glass and so on, with the glass becoming a defacto streaming video display to watch burglars/traffic etc. Interesting concept, smashing a pane of year glass - would it release all that energy as an e=mc2 blast?? https://sf-encyclopedia.com/entry/slow_glass
denton-scratch
You can model refraction using waves in a water-tank; the waves slow down in shallow water, and speed up in deeper water. You can do things like make lenses.
So I suppose a near-zero refractive index would be modeled by a region of "infinite depth". Is there a relationship between wavelength and depth, such that for a given wavelength, there is some finite depth that behaves for practical purposes as if it were infinite?
jeffparsons
I have no physics education beyond high school, but I briefly became obsessed with the idea of efficient collimation of light. I eventually concluded that it must be impossible, because it would imply a global decrease in entropy, much like Maxwell's demon.
But this article seems to suggest that it might still be possible. Have I misread it? Or maybe misinterpreted — e.g. maybe "efficient" would cover a system that does require energy input, and just doesn't require discarding a lot of the input light?
Can anyone with more of a clue comment on this?
I find the idea exciting because I can imagine endless practical uses for an efficient collimator.
mnw21cam
It'd be balanced by the difficulty in getting the light into the material in the first place. You're right, you can't change the laws of physics. Light will only enter an object with lower refractive index if its angle of incidence is close enough to straight on that the beam coming through into the material has somewhere to go that satisfies Snell's law. When the object's refractive index is near zero, the object will only accept incoming light across an extremely narrow angle. This solves your concerns about having a global decrease in entropy.
You can fudge this by making the incoming surface of the object match the shape of the incoming waves. Then all the light will be allowed in. However, that surface will only match the shape of the incoming waves for light originating from a tiny point in space. If your light source is larger than a tiny point in space, then the light coming from the larger area will bounce off the material and not enter it. In effect, the magic material will magically collimate a light source, but only if the light source is already fairly magic, and in that case a simple lens would suffice.
jeffparsons
Thanks for the explanation.
It makes me a little sad, but maybe it's for the best; efficient collimation of light would offer enormous destructive potential for $cheap, and that's not something the world needs any more of.
dekhn
it sounds like etendue
cbondurant
The first application that comes to mind for me, and of course I am not a quantum physicist so its entirely possible that my conceptions here are misguided, would be with creating even more perfectly coherent beams of laser light for use in the high precision environments of quantum laser experiments.
Possibly either at more affordable rates or to allow for an extremely temporally coherent wavefront in other shapes besides just the beam that is most commonly used? I could easily believe that there are potential experiments that are just waiting for the right technology to be viable.
carterschonwald
This is cool. This is the sort of stuff I like to see on the internet.
FireInsight
I'm on my phone, and the privacy banner takes so much space I just see the title, a divider, and then the banner. I started reading the banner thinking it was the article...
gcanyon
For anyone who's interested, Fermilab has done several videos on this topic as well that I found highly informative:
Jeff_Brown
I don't understand why this does not imply the possibility of faster than light communication. If you had a long tube with a zero refractive index, couldn't you type Morse code into one end, by simply turning the signal on and off, and have the receiver receive the message immediately?
edgineer
3blue1brown's video covers this. The effect of "changing speed" is a change to the light's wavelength, not the speed of an impulse through the material--imagine taking a standing wave and pushing the peaks and valleys up and down so the wavelength looks really long. And since the frequency is unchanged, there's your appearance of high speed. This happens because the material changes the phase of the wave continuously.
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For anyone who hasn't studied electrodynamics or advanced optics, Grant Sanderson (3Blue1Brown) recently made a detailed video explaining what the refractive index really means, probably the clearest reasonably accessible explanation of this topic anywhere to date:
https://www.youtube.com/watch?v=KTzGBJPuJwM
https://www.youtube.com/watch?v=Cz4Q4QOuoo8
The first video is really a tour de force. Too bad it saw such limited traction here: https://news.ycombinator.com/item?id=38482549