Yeah, it didn't help anything. But I don't have to invoke QFT and EM to explain why: 1) He's saying the light moves the electrons and the electrons set up a 2

Yeah, it didn't help anything. But I don't have to invoke QFT and EM to explain why: 1) He's saying the light moves the electrons and the electrons set up a 2nd wave that interferes with the original to produce a lower frequency. That's simple enough. But then he says the lower frequency wave travels slower. Eh, what? We are try to explain is why light travels slower? Ergo, you can use "because it travels slower" in your explanation. 2) That said, the lower frequency does travel slower because light gets split into a spectrum by the glass prism. To me it seems a satisfying answer would not only explain why it got slower, but why slowdown depends on frequency.