Why does light slow down in water?
Fermilab3 HN recommenders · recommended in 3 months, 2020–2023
explainer · general · evergreen · 2020–2023 on HN
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.
HN commenters link it to correct the widespread misconception about why light slows in a medium.
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4 comments from Hacker News
For anyone who's interested, Fermilab has done several videos on this topic as well that I found highly informative: ▶ this video link
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
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
> 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