6 ms·

For anyone who hasn't studied electrodynamics or advanced optics, Grant Sanderson (3Blue1Brown) recently made a detailed video explaining what the refractive in

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=KTzGBJPuJwM https://www.youtube.com/watch?v=Cz4Q4QOuoo8 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 https://news.ycombinator.com/item?id=38482549

hadlock 3y ago
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 3y ago
  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.
  - mikewarot 3y ago
    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.
    - Kranar 3y ago
      Phase kicks don't change the speed of a wave. If you setup a similar experiment with sound or water where you constructed an interference pattern that kicked back the phase of the wave, the wave wouldn't slow down. If you want an actual non-layman explanation for why light slows down ib a medium you need to express the light and medium interaction in terms of polaritons. Of course this is incredibly difficult to do so for even simple cases, so alas a whole suite of simplified explanations exist that seek to explain some aspect of the situation while failing to explain others. https://en.m.wikipedia.org/wiki/Polariton https://en.m.wikipedia.org/wiki/Polariton
      - deely3 3y ago
        Could you elaborate please? So, there no slowdown by phase shift at all? But.. are you sure, because it looks like the phase shift is the real effect and logically speaking result of the phase shift will be the effect that look like slowdown for outside observer. Also, how can you simulate similar effect in water and sound if there no sound polarization?
        mikewarot 3y ago
        The phase shift is dependent on both the resonance frequency of the medium and the incident light. In the case of X-rays, the refractive index can go below 1, resulting in total external reflection for a few tenths of a degree. Even in this case, the speed of the X-ray itself stays below that of C, even though the "wavelength" appears to be longer than in vacuum.
        3y ago
- martincmartin 3y ago
  No need to dig out the books, they're available online, well formatted and a pleasure to read: https://www.feynmanlectures.caltech.edu/ https://www.feynmanlectures.caltech.edu/ https://www.feynmanlectures.caltech.edu/II_32.html https://www.feynmanlectures.caltech.edu/II_32.html
dcreater 3y ago
Grant has singlehandedly taught me more than all of my elite education combined. Man is an absolute treasure
platz 3y ago
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?
SAI_Peregrinus 3y ago
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. [1] https://youtu.be/SS2AbZVdk2A?si=5KTvkX-KVnxRUN7G https://youtu.be/SS2AbZVdk2A?si=5KTvkX-KVnxRUN7G

hadlock 3y ago

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 3y ago
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.
- mikewarot 3y ago
  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.
  - Kranar 3y ago
    Phase kicks don't change the speed of a wave. If you setup a similar experiment with sound or water where you constructed an interference pattern that kicked back the phase of the wave, the wave wouldn't slow down. If you want an actual non-layman explanation for why light slows down ib a medium you need to express the light and medium interaction in terms of polaritons. Of course this is incredibly difficult to do so for even simple cases, so alas a whole suite of simplified explanations exist that seek to explain some aspect of the situation while failing to explain others. https://en.m.wikipedia.org/wiki/Polariton https://en.m.wikipedia.org/wiki/Polariton
    - deely3 3y ago
      Could you elaborate please? So, there no slowdown by phase shift at all? But.. are you sure, because it looks like the phase shift is the real effect and logically speaking result of the phase shift will be the effect that look like slowdown for outside observer. Also, how can you simulate similar effect in water and sound if there no sound polarization?
      - mikewarot 3y ago
        The phase shift is dependent on both the resonance frequency of the medium and the incident light. In the case of X-rays, the refractive index can go below 1, resulting in total external reflection for a few tenths of a degree. Even in this case, the speed of the X-ray itself stays below that of C, even though the "wavelength" appears to be longer than in vacuum.
      - 3y ago
martincmartin 3y ago
No need to dig out the books, they're available online, well formatted and a pleasure to read: https://www.feynmanlectures.caltech.edu/ https://www.feynmanlectures.caltech.edu/ https://www.feynmanlectures.caltech.edu/II_32.html https://www.feynmanlectures.caltech.edu/II_32.html

dcreater 3y ago

Grant has singlehandedly taught me more than all of my elite education combined. Man is an absolute treasure

platz 3y ago

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?

SAI_Peregrinus 3y ago

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. [1] https://youtu.be/SS2AbZVdk2A?si=5KTvkX-KVnxRUN7G https://youtu.be/SS2AbZVdk2A?si=5KTvkX-KVnxRUN7G