1.3M views · Feb 20, 2019 · 10:24 · 3 HN recommenders · 4 mentions · discussed 2020–2023
Fermilab's Don Lincoln explains why light travels slower in water and glass. He knocks down common wrong explanations such as absorption and re-emission, then gives the wave-mechanics reason tied to refractive index.
Why Hacker News points people here: HN commenters link it to correct the widespread misconception about why light slows in a medium.
“See this video for an explanation of why light travels slower through a medium with an index of refraction greater than one.”HN comment, 2023
“For a video to address this misconception of the speed of light being constant”
Corrects the misconception about light slowing in media.
For a video to address this misconception of the speed of light being constant, and it just appears to slow down due to ping-pong, I recommend "Why does light slow down in water?" by Fermilab: this video
“See this video for an explanation of why light travels slower through a medium with an index of refraction greater than one.”
Why light is slower in media with higher refractive index.
I had thought because light was absorbed and re-emitted but that and scattering ideas are incorrect. See this video for an explanation of why light travels slower through a medium with an index of refraction greater than one. this video
“Fermilab has done several videos on this topic as well that I found highly informative”
Fermilab videos on refractive index.
For anyone who's interested, Fermilab has done several videos on this topic as well that I found highly informative: this video link
“This is wrong. Two sources [0][1].”
Source refuting the absorb and re-emit explanation.
> 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] this video [1] link
