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Why does light slow down in water?
About this video
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 it appears in the archive
HN commenters link it to correct the widespread misconception about why light slows in a medium.
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From the Hacker News discussion
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: [VIDEO]
Read original comment ↗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. [VIDEO]
Read original comment ↗For anyone who's interested, Fermilab has done several videos on this topic as well that I found highly informative: [VIDEO] [LINK]
Read original comment ↗> 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] [VIDEO] [1] [LINK]
Read original comment ↗