1 HN user recommended this explainer1 month2018–2021score 1
Years with an HN recommendation, 2007–2026
HN cites it whenever the popular absorb-and-re-emit explanation comes up, as a source for the wave-interference account.
Professor Mike Merrifield of Nottingham explains in a mostly uncut conversation why light travels slower in glass. He treats light as a wave and describes how the material's electrons produce secondary waves that combine to slow it.
“This reminds me of this Sixty Symbols video which explains why light is slower in media such as glass than in a vacuum” (HN, 2019)
This reminds me of this Sixty Symbols video which explains why light is slower in media such as glass than in a vacuum: [this video] The incoming light wave causes an avalanche of secondary waves through electromagnetically perturbing the individual atoms, and the superposition of all these results in a wave that seems to travel slower than the speed of light.
Explains why light is slower in glass via secondary waves
> Light seems like it slows down in dense media, but light always propagates at the fastest possible speed for any physical thing. The slow down occurs when the light gets briefly absorbed by atoms and then re-emitted in the same direction. Someone please correct me if I'm wrong, but Sixty Symbols (aka nottinghamscience) did at least 2 videos on this where they expressly pointed out that this is widely pushed but totally incorrect? [this video]
Says absorption and re-emission explanation is incorrect
thanks! When I wrote that line I think I went with the top result of physics.stackexchange, but I'm now convinced that the phase velocity explanation is better. Here is my updated text: <p>While individual waves of light always move at c, light seems to move slower in dense media. This is because as the light wave propagates through a medium it produces ripples that <a href=" [this video] in a way that slows the group velocity of the light wave.</p>
Source for phase velocity explanation of slowed light
Recently I watched Sixty Symbols video on why light is slower in glass and there are multiple explanations on it. What struck me was what prof. Michael Merrifield said at the end of his explanation: [this video] On the question about what is the reality, what explanation is the true one, Merrifield said that the Math works on all of the explanations and what those explanations do is simply to model the behaviour of nature and not necessarily reflect the reality. That's how Newtonian physics and relativistic physics are both correct models, tools to model nature and simply can be used to whenever suitable. Wouldn't that mean that mathematics is just an invented tool to reason about physical models?
Multiple explanations; math models rather than reality
downvoted because time traveling through a medium vs traveling through curved space are not anywhere near the same underlying principles[0]. The implication being suggested is - if I was trapped in a block of plastic or... underwater, I would experience time different compared to someone standing on land just because "c" is slower. Clearly not the case. To be specific, when "the speed of light" is discussed like this, it usually means the "speed of causality"[1] which is the underlying meaning of "c" in relativity. The speed of causality through a medium (i.e. water) is the same as a vacuum if spacetime curvature is the same. [0] [this video] [1] [link]
Cited on light slowing in media versus curved space
Absorption and re-emission are probabilistic, so you'd expect experiments to show a spread distribution of propagation times, deflection angles, etc that aren't actually observed. The explanation gets into looking at light as a wave. [this video] [link]
> Well, actually, the speed of light is constant, it just takes time to ping-pong around a lattice/gas. Photons hit particles, get absorbed, and re-emitted, which causes an effective slowdown at a larger scale. A gas is just vacuum with particles floating around in it. When photons travel between those particles, they are traveling at c. The effective slowdown is determined by the lattice/gas, and we denote this slowdown with the refractive index... This is wrong. Two sources [0][1]. [0] [link] [1] [this video]
Source refuting the ping-pong absorption explanation
Both ideas are incorrect. Bouncing around would be random and the outgoing light would be scattered (as in matte reflections) in all directions. Absorbtion simply destroys the original light. If photons are mostly absorbed, we call that “a wall” or ”a fog”, not “a transparent medium”, which to be transparent shouldn’t have electrons whose energy level differences correspond to a photon’s wave length. The transparency and slowdown effect is a complex interaction between fields in the medium, which I honestly don’t have enough courage to retell. [this video]
Transparency and slowdown come from field interactions in the medium
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