But why would light 'slow down'? | Optics puzzles 3

How the index of refraction arises, and why it depends on color.
An equally valuable form of support is to simply share the videos.

Looking Glass Universe videos on the index of refraction:

Much of this video is based on the following Feynmann lecture

The explanation for why the phase of a wave produced by a plane of oscillating charges is a quarter phase behind the wave of a charge in the center of that plane, and hence a quarter phase behind that of a light wave inducing the oscillations, is given in the previous chapter:

Sections:
0:00 - The standard explanation
3:14 - The plan
5:09 - Phase kicks
8:25 - What causes light?
13:20 - Adding waves
16:40 - Modeling the charge oscillation
20:59 - The driven harmonic oscillator
26:57 - End notes

Thanks to these viewers for their contributions to translations
German: Ole
Hebrew: Alon Altman, Omer Tuchfeld
Hindi: Saurabh
Portuguese: @doryhelioaires
Urdu: Momini0

------------------

These animations are largely made using a custom Python library, manim. See the FAQ comments here:

All code for specific videos is visible here:

The music is by Vincent Rubinetti.

------------------

3blue1brown is a channel about animating math, in all senses of the word animate. If you're reading the bottom of a video description, I'm guessing you're more interested than the average viewer in lessons here. It would mean a lot to me if you chose to stay up to date on new ones, either by subscribing here on YouTube or otherwise following on whichever platform below you check most regularly.

[Rubrickety]

Part of what makes Grant such a superlative educator is how he treats the things he leaves out or simplifies. He always points them out, explains why he's skipping over them, and provides a hint as to what including them would look like. It's never "It's too complicated, just don't worry about it."

[andycgn]

If there was a Nobel Prize for explanations, you would be my canditate. Thank you for taking the trouble to explain things so profoundly and vividly. You are my hero.

[claudiooton]

I teach optics in the University, and I can say that you nailed this one, really the best explanation I've ever seen on the matter. I'm sure Feynman would hug you if he could see this video.

[rectorsquid]

I have always regretted my lack of math education. The equations in the video barely make sense - the algebra makes sense but the differential equation stuff is a language I just don't speak - so seeing the visuals to explain these things make them accessible to me. Thanks. And those visuals are absolutely stunning!

[lakshya]

this is the clearest explanation I ever heard. It's like 15 optics lectures condensed in a single video. Absolutely mind blowing...

[aaaaa]

I always wondered what Feynman's lectures would have been like if he taught them with today's technology, and man this is it. The world is going to have some brilliant physicists and mathematicians thanks to you.

[tahsinshafin]

I'm genuinely telling you Grant, for all the human beings seeking the absolute deepness of reality asking every bit of "Why", *you are an unprecedented blessing.*

[laxminarayanbhandari]

"so, everything was just a harmonic oscillator?"
"always has been."

Great video as always. Waiting for the next part!

[paparapiropip]

After a double major in math and physics, I'm doing a PhD in physics. A great part of it deals with light-matter interaction. I must say this is a beautiful representation of a phenomenon that most of the time physicists "sort of" imagine. Thanks for such a great way of visualizing physics! And thanks for giving our dear harmonic oscillator the visibility it deserves!

[NiMareQ]

I don't think there is a material on youtube or the internet explaining this stuff in such detail and so comprehensibly. I am blown away. It instantly clicks!

[HuygensOptics]

The animations in this video are clarifying so many details that are very hard to explain in any other way. Fantastic, I loved every second of it!

[squidsword]

Being able to boil down complicated phenomena to the bone this elegantly... None of us would have ever developed the background to absorb this without your help. You’ve gifted so many people the fascination of understanding how reality works

[tirterra]

I was genuinely thinking about working on prisms an hour ago as an electromagnetism exercise, this is too perfect to be true.

[abrahanpinedo]

This video reminded me of a talk I had with some friends at CERN, where I told them that we didn't really understand why light refracts, the typical explanations weren't enough. When I looked into it myself, I remember reading the response in the Feynman Lectures and being more or less pleased with what it said. But now with the video and animations everything clicked! From a question I had asked myself ten years ago, it is an incredible sensation! Thank you Grant.

[xminty]

22:11 my favorite 3b1b moment
thank you Grant for your works it has been monumental to my studies

[AstroMattWood]

I teach a university course in Waves/Acoustics/Optics, and this video is now assigned viewing. Amazingly well done - thank you.

[Anjihyu]

The single best channel on this entire site. I wish I had money to spare, because you, mister, deserve it more than anyone. Brilliant video, brilliant editing, brilliant visuals, brilliant explanation.

[minhhieuao]

Love your animation, please never stop doing this🙏

[tHaTsWhAtI.mSaYiNg]

This may be one of the first 3b1b videos I actually understood, wonderful job.
Also it is kind of adorable thinking that Grant approached his niece like 'can I push you in a swing for a math video, but like, really badly'

[jakeadams]

13 minutes in and you’ve already granted 3 revelations in my understanding of our world that would have otherwise taken years for me to understand… you have a very special talent and I thank you massively.