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But why would light "slow down"? | Visualizing Feynman's lecture on the refractive index

3Blue1Brown · explainer · Watch on YouTube

Fundamental physicsEveryday physics & science demos

What it is

3Blue1Brown visualizes Feynman's lecture on the refractive index, explaining why light appears to slow down in glass and why the effect depends on color. It builds the idea from waves produced by oscillating charges in the material.

Why Hacker News recommends it

HN considers it among the clearest accessible explanations of what refractive index actually is and how light moves through transparent matter.

  • 5 people recommended it
  • 5 different months
  • 1 strong recommendations
  • first 2023, last 2026

What HN commenters said

  1. “probably the clearest reasonably accessible explanation of this topic anywhere to date”

    Clear, accessible explanation of what refractive index really means.

    jacobolus, 2023-12 · Strong recommendation · in “What is a zero refractive index material?”

  2. “I found this 3blue1brown video to be a fascinating explaination on how light gets transmitted by transparent material”

    Explains how light is transmitted through transparent material.

    ithkuil, 2024-01 · Recommendation · in “Zeiss's "Holocam" turns glass windows into cameras”

  3. “here’s a well done video explaining the topic of the apparent changes of light’s “speed” through materials”

    Explains apparent changes of light's speed in materials.

    Modified3019, 2024-04 · Recommendation · in “Does light have an infinite lifetime?”

  4. “There's a very good video by 3Blue1BRown that explains this kind of weird concept way better than I could”

    Explains how secondary fields slow apparent light propagation in materials.

    tatjam, 2025-02 · Recommendation · in “But good sir, what is electricity?”

  5. “For another fantastic explainer on optics, see 3Blue1Brown's video on refraction”

    Fantastic explainer on optics and refraction.

    _jayhack_, 2026-01 · Recommendation · in “Cameras and Lenses”

Excerpts from Hacker News comments, linked to the originals. Summaries and labels were written by a language model from those comments.

Topics: refractive index, 3blue1brown, feynman, light, physics, waves

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