4 ms·
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 e
by 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 agoI 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 agoPhase 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 agoCould 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 agoThe 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.
- michaelt 3y ago> logically speaking result of the phase shift will be the effect that look like slowdown for outside observer. If there was a phase shift but no slowdown, wouldn't we expect individual photons to propagate through a 10km loop of fibre optic cable without any slowdown, which would be measurable?
- Kranar 3y agoYou would certainly see an apparent slowdown due to phase shift if you consider a continuous wave that spans the entire medium, yes for sure and so this explanation and model does have merit, I don't want to dismiss it entirely. But this model won't be useful for understanding pulses of light through a medium. For example if I emit a very short pulse of light through a medium and measure how long it takes the head of the pulse to traverse the medium, using this model won't work. A phase shift won't move the head of a pulse of light backwards, it will just adjust the amplitude of the head as it propagates through the medium. You still need a way to explain how it is that the head of the pulse actually takes longer to travel through the medium and that's not something a phase shift can explain.
- rocqua 3y agoThe understanding of a refractive index only cares about 'phase speed' in steady state. At least, that is what the video claims to be about. For that case, phase kicks are a sufficient explanation no? I can imagine things get really weird with pulses, but that's out of scope for the video.
- gpsx 3y agoI'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".
- Tazerenix 3y agoThe 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.
- MichaelZuo 3y agoWhat does it mean for something to oscillate electromagnetically and mechanically simultaneously? Doesn't mechanical oscillation already occupy all three spatial dimensions plus a time dimension?
- zmgsabst 3y agoThe phonon oscillates both the electrons and the EM field. A photon is EM only. So you’re correct that it occupies all four dimensions, but we’re discussing an excitation in one field (photon) changing to an oscillation in multiple (phonon) and then back to only one field (photon).
- MichaelZuo 3y agoCan you explain this in a step by step manner? I really can't quite grasp how the photon/phonon switches back and forth like this.
- zmgsabst 3y ago1. The photon moves through free space, where only the EM field is disturbed. 2. The photon enters a material, where the disturbance in the EM field couples to the electron waves — creating a phonon. 3. The phonon travels through the material, as an oscillation in both electrons and EM. 4. The phonon reaches the edge of the material, where there are no more electrons and reverts to a wave of just EM — a photon. 5. The photon continues in free space. The reason this happens is the photon changes the electron behavior, which in turn changes the photon behavior. This is because EM interacts with charged particles like electrons. For the time it is in a material, a EM wave can’t be separated from the behavior of the electrons present: both and their mutual interaction are required to explain what happens. That disturbance of both is “coupled” — and called a phonon.
- dustingetz 3y agovisualized well in “Picture Book of Quantum Mechanics” ch 1 (a fairly hardcore physics text that does indeed have many diagrams)