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Why does light slow down in water?

Name: Why does light slow down in water?
Description: 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.

Fermilab Explainers

Open on YouTube First HN thread

3 HN users recommended this explainer in 3 different months between 2020 and 2023.

2007–2026

3 recommendations 1 citation HackerTube score 3

HN commenters link it to correct the widespread misconception about why light slows in a medium.

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.

Pull quote: “See this video for an explanation of why light travels slower through a medium with an index of refraction greater than one.” (HN, 2023)

Also tagged: refractive index, speed of light, optics, physics misconceptions, wave mechanics, refraction

What Hacker News said

4 labeled comments from the HN archive; up to 20 are kept per video, favoring different commenters. Each one links back to the original thread.

Recommends gugagore December 2020 under Well, Actually – Physics Answers

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]

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Recommends djmips January 2023 under Ask HN: Concepts that clicked only years after you first encountered them?

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]

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Recommends gcanyon December 2023 under What is a zero refractive index material?

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]

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Citations, criticism and jokes

Cites it vecter December 2020 under Well, Actually – Physics Answers

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]

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