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

3Blue1Brown · 2.9M views · 2023-11-30 · 28:34

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Overview

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.

On Hacker News

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

Recommended by 5 HN people across 5 months · HN mentions 2023–2026

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What people said

jacobolus · 2023-12 · Strong recommendation

For anyone who hasn't studied electrodynamics or advanced optics, Grant Sanderson (3Blue1Brown) recently made a detailed video explaining what the refractive index really means, probably the clearest reasonably accessible explanation of this topic anywhere to date: [VIDEO] [LINK] The first video is really a tour de force. Too bad it saw such limited traction here: [LINK]

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_jayhack_ · 2026-01 · Recommendation

Great article. For another fantastic explainer on optics, see 3Blue1Brown's video on refraction: [VIDEO]

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tatjam · 2025-02 · Recommendation

I'm not a physicist, but as far as I know, outside of general relativity electromagnetic perturbations always travel at the speed of light (i.e. to affirm that photons always travel at c is correct). It's only after the fields interact with electrical charges (atoms and their electrons for example) that a secondary field is induced as these charges begin to oscillate. This field will add over the original field, "shielding" an external observer from the original oscillation and apparently slowing down the propagation of electromagnetic waves. There's a very good video by 3Blue1BRown that explains this kind of weird concept way better than I could: [VIDEO]

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Sources

Video by 3Blue1Brown, played through YouTube.

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