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

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: ▶ this video link The first video is really a tour de force. Too bad it saw such limited traction here: link

Great article. For another fantastic explainer on optics, see 3Blue1Brown's video on refraction: ▶ this video

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: ▶ this video

For those curious about what the above poster is talking about, here’s a well done video explaining the topic of the apparent changes of light’s “speed” through materials 3Blue1Brown - But why would light "slow down"? | Optics puzzles 3: ▶ this video

I found this 3blue1brown video to be a fascinating explaination on how light gets transmitted by transparent material: ▶ this video

Great article! I particularly like this paragraph: >It’s important to note that while the charge equalization process is fast, the drift of individual electrons is not. The field propagates at close to the speed of light in vacuum (circa 300,000 km/s); individual electrons in a copper wire typically slither at speeds measured in centimeters per hour or less. A crude analogy is the travel of sound waves in air: if you yell at someone, they will hear you long before any single air molecule makes it from here to there. So basically electricity flows like a Newton's cradle. But this leaves one nagging question: what is the nature of the delay? This question also arises when considering the microscopic cause of index-of-refraction for light[1]. If you take a simple atom, like hydrogen, and shine a light on it of a particular frequency, I understand that the electron will jump to a higher energy energy level, and then fall back down. But what governs the delay between these jumps? And also, how is it that, in general, light will continue propagating in the same direction? That is, there seems to be some state-erasure or else the electron would have to "remember" more details about the photon that excited it. (And who knows? Maybe the electron does "remember" the incident photon through some sort of distortion of the quantum field which governs the electron's motion.) The same question applies to electron flow - what are the parameters that determine the speed of electricity in a co