Why does light slow down in water?

There are many mysteries of physics for which you can find explanations online and some of those explanations are wrong. In this video, Fermilab’s Dr. Don Lincoln takes on the mystery of why light travels slower in water and glass. He lists a few wrong explanations and then shows you the real reason this happens.

[stefanhensel]

It took me 55 years and 10 minutes to understand. Thank you!

[durragas]

Anyone else coming back to this video every few months because you forgot how it happens and you're thinking about it again?

[arthurrae]

Glad you recorded: Why does light bend when it enters glass. Recommend people view that one first, to assist with understanding this video.

[ThomasJr]

Another easy to follow video. The more we study and watch these videos, the more knowledgeable we become. Thank you Dr Don.

[theCodyReeder]

Excuse me while I pick my jaw up off the floor.

[JoaoPedro-piee]

I'm an electronics engineer and we study Maxwell's and wave equations, of course. Optics is taught in high school. I've never really stopped to think how these two relate and your video made it crystal clear. Very interesting!

[TonyJohns-wigo]

Fantastic video! There's one key piece still missing for me: "light still moves at the speed of light, but the waves from the electrons move at a different speed, and the combined wave moves slower than light would..." My question is: how can the waves from the electrons move at a different speed? All electro-magnetic radiation moves at the speed of light... how can the waves from the electrons be slower? They'll be moving in a different direction, but not slower. No?

[mickeyfilmer]

I have had the wrong explanation since 1971-when I studied physics and we were told the guff about the density of the glass being the reason -what a shame teachers then didn't have to have majored in their subject - thank you for putting me right. I'm mid 60's and still learning basics thanks to people like yourself !

[philipnoonan]

Thanks, Dr. Don Lincoln. I've taught refraction for years. I'm talking pre-internet. But today, via your video, I really got to the heart of the reason. I totally dig your engaging and warm style and also your super cool physics wardrobe!

[paritosh]

5:32 "...when an atom has been absorbed by a photon..."

Did I miss something?

[TheSkystrider]

But why the change of direction and how does it remember the original angle to go parallel to original direction?

[methanelau]

Thank you sir. I have a question at 9:19, “The wave from the electrons move at a different speed”, why it is different?

[haimbenavraham]

it's a well-known fact I learned in Ireland ... light slows down in water because it's thirsty.

[tariqchouaiby]

Damn one of the most straight forward explenations i've seen on youtube, while also explaining common misconsceptions: brilliant, definitly a sub.

[Fazzel]

Something I found interesting is a physics lab I had in school where we measured the index of refraction of opaque objects. Up until that time I thought only transparent object had an index of refraction.

[JaimeWarlock]

I was taught about fifty years ago in physics that the atoms absorbed and emitted the light in same direction (the second wrong belief). For various reasons (doesn't explain speed of light in different materials very well) it never felt right. When I saw the title to your video, decided to get a better understanding of the process. Surprise ... I was taught wrong. This explanation makes much better sense.

[jongtaekim]

Thank you! I graduated physics department, but for the first time I come to understand why light travels slowly in water. Thank you for your nice video!

[alexanderk.]

Professor Fermilab --- I love you ( and I am 56 years old)! Please continue!

[MaDrung]

We've learned about this in optoelectronics class and had to derive it. I was very happy about learning about it. Although I forgot what is the exact reason for the light bending, as opposed to just "illusionary" slowing down. Snell law is all I remember :P

EDIT: Have watched your next video explaining the light bending. as expected, same phenomenon.

[studyhard]

9:05 ✌️ watch from here if u r in hurry, the actual reason I clicked on this video
Thank u sir ji, mujhe smj aa gya!!