HackerBut why would light "slow down"? | Visualizing Feynman's lecture on the refractive index
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Further reading 1
Caption notes
Superposing the incoming wave with radiation from driven charges shifts the phase of the resultant wave. Repeated through the medium, those shifts give the crests a lower phase velocity.
Creator’s caption track ↗Original passages · 2
a wave that's simply traveling slower, oscillating up and down with the same frequencySo when you add them together, you get this little phase shift.