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    Why does light slow down in water?

    Fermilab

    3 HN recommenders · recommended in 3 months, 2020–2023

    YouTube

    10:24 · 1.3M views · Feb 20, 2019

    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

    1. @djmipsJan 2023· in “Ask HN: Concepts that clicked only years after you first encountered them?”

      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

      RecommendsView on HN
    2. @gugagoreDec 2020· in “Well, Actually – Physics Answers”

      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

      RecommendsView on HN
    3. @gcanyonDec 2023· in “What is a zero refractive index material?”

      For anyone who's interested, Fermilab has done several videos on this topic as well that I found highly informative: this video [LINK]

      RecommendsView on HN
    4. @vecterDec 2020· in “Well, Actually – Physics Answers”

      > 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]

      CitesView on HN