# Speed of light in water and vacuum **URL:** \ **Category:** Factual Questions\ **Created:** [October 24, 2001, 4:56pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335 "2001-10-24T16:56:51Z")\ **Posts on this page:** 13\ **Page:** 1
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 24, 2001, 4:56pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/1 "2001-10-24T16:56:51Z")
> [@](#): > > _Cecil Adams wrote in today’s column:_ > Not exactly–nothing can travel faster than the speed of light in a vacuum, _c_. However, in translucent media, notably water, light travels much slower, at maybe 75 > percent of _c_. A beta particle traveling through air, say, moves considerably faster than light traveling through water. Since my question does not relate specifically to today’s column, I’m asking it in GQ. Let’s assume we have a big vacuum chamber with a big tank of water at one end. We shoot some light through the water and into the vacuum. As **Cecil** says, the light is at 75% of _c_ when it passes through the water. What happens when it reaches the vacuum? Does it remain at 75%_c_? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 24, 2001, 5:08pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/2 "2001-10-24T17:08:38Z")
Nope…it’s back up to the full speed once it returns to the vacuum. However, I am curious to know if the photon accelerates up to full speed (i.e. 76%, 77%, 78%, etc.) or if it just goes from 75% to 100% without bypassing all of the in-between speeds (my vote is for the second option)? ---
**Author:** ![Johnny\_L.A](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/johnny_l.a/32/1084_2.png) [@Johnny\_L.A](https://boards.straightdope.com/u/Johnny_L.A)\ **Post date:** [October 24, 2001, 5:13pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/3 "2001-10-24T17:13:43Z")
> [@](#): > > Nope…it’s back up to the full speed once it returns to the vacuum. I thought I had heard that somewhere. Okay, next question: Where does it get the energy to speed up? Or does it get “something for nothing”? ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 24, 2001, 5:29pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/4 "2001-10-24T17:29:03Z")
> [@](#): > > \*Originally posted by Johnny L.A. \* > \*\*Okay, next question: Where does it get the energy to speed up? Or does it get “something for nothing”? \*\* Just a WAG… I don’t think the photon loses energy when entering a different medium. It is the resistance of the medium that slows it down. For example, imagine you are walking across a room at a perfectly steady pace and have X speed. Now I stand in front of you and try to hold you back. Assume you maintain exactly the same energy output. Being much stronger than I am you keep moving forward but now you are moving forward more slowly. Once I step out of the way it’s back to full-speed for you but you haven’t gained in power…your power output (as it were) has remained steady throughout. ---
**Author:** ![sailor](https://avatars.discourse-cdn.com/v4/letter/s/a587f6/32.png) [@sailor](https://boards.straightdope.com/u/sailor)\ **Post date:** [October 24, 2001, 5:31pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/5 "2001-10-24T17:31:10Z")
Who said faster light has more energy? Sound travels faster in concrete than in water. If you make a sound in the pool when it gets to the concrete it will speed up _instantly_… or would you expect it to slowly pick up speed? [sub]Yes, I know the difference between light and sound, I am not blind and I am not deaf; I am just dumb.[/sub] ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 24, 2001, 5:39pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/6 "2001-10-24T17:39:58Z")
Actually I wonder what is happening when a photon travels through a medium other than a vacuum that makes it slow down? I don’t know if my example above makes any sense but an answer to the following might shed some light (I don’t know if either of the below are correct and am asking more than speculating here). Two possibilities: 1. The photon while traveling through the medium strikes other atoms along the way which cause it to ‘ricochet’ off in some other direction. The speed of the photon is always constant but this zig-zagging its way through a material adds time so, as measured from the outside, the photon has taken longer to travel from point A to point B in the medium. The apparent loss in speed is due to the photon travelling a greater distance than a straight line from A to B. 2. The photon while travelling through a given medium strikes an atom and is absorbed. The atom then re-emits the photon and it continues on its merry way going from point A to point B. It is the latency introduced in the absorbtion and re-emission of the photon that introduces a lag appearing to an observer as a slower speed in the medium. As long as the photon is ‘between’ atoms its speed is still that of light in a vacuum. Those are the two possibilities I can think of but I don’t know if there are others or if either or both of those are correct. Anyone know for sure? [sub]I have a sneaking suspicion a few people around here do.[/sub] ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [October 24, 2001, 6:52pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/7 "2001-10-24T18:52:54Z")
> [@](#): > > Whack-a-Mole wrote: > > 1. The photon while travelling through a given medium strikes an atom and is absorbed. The atom then re-emits the > photon and it continues on its merry way going from point A to point B. It is the latency introduced in the > absorbtion and re-emission of the photon that introduces a lag appearing to an observer as a slower speed in the > medium. As long as the photon is ‘between’ atoms its speed is still that of light in a vacuum. You got it. A photon always travels at c. See the following for a good animation of light delay from absorbtion and reemission. [http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/em.html](http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/em.html) ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 24, 2001, 7:13pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/8 "2001-10-24T19:13:42Z")
> [@](#): > > \*Originally posted by Ring \* > \*\*[http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/em.html](http://www.glenbrook.k12.il.us/gbssci/phys/mmedia/waves/em.html) \*\* [Off Topic] Interesting…I went to Glenbrook South high school. Do you have something to do with that school or is it just a weird coincidence (from my perspective) that you happened to post a link to there? [/Off Topic] ---
**Author:** ![Whack-a-Mole](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/whack-a-mole/32/141_2.png) [@Whack-a-Mole](https://boards.straightdope.com/u/Whack-a-Mole)\ **Post date:** [October 24, 2001, 7:31pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/9 "2001-10-24T19:31:01Z")
Potential Hijack (hope JLA doesn’t mind)… Why does an atom re-emit an absorbed photon opposite of the side the photon hit it from such that light passes straight through? Ok, I guess light is bent (which you can feel free to explain as well) but I would guess that light would be re-emitted all over the place in random directions. As I understand it a photon hits an atom and is absorbed thus adding energy which moves an electron to a higher valence (I think that’s the right word). The electron then drops back into a lower state and emits a photon in the process. I would guess the electron is somewhat randomly whizzing around the atom and would therefore drop back into a lower state at some random place rather than _always_ at a spot that emits a photon which will continue on a path as if the atom had never been there in the first place. It would seem that my guesses are wrong since I can look through a window but what’s the deal here? ---
**Author:** ![robo](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@robo](https://boards.straightdope.com/u/robo)\ **Post date:** [October 24, 2001, 7:33pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/10 "2001-10-24T19:33:28Z")
_However, I am curious to know if the photon accelerates up to full speed (i.e. 76%, 77%, 78%, etc.) or if it just goes from 75% to 100% without bypassing all of the in-between speeds (my vote is for the second option)?_ I think Ring confirmed it best regarding the absorption/emission process. Anyway, it’ll never realistically be the case where you have a total vacuum right next to a non-vacuum. The density of matter will nearly always change gradually so the speed of the beam of light will gradually increase though it may do so very fast. ---
**Author:** ![robo](https://avatars.discourse-cdn.com/v4/letter/r/bb73d2/32.png) [@robo](https://boards.straightdope.com/u/robo)\ **Post date:** [October 24, 2001, 7:44pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/11 "2001-10-24T19:44:16Z")
_Why does an atom re-emit an absorbed photon opposite of the side the photon hit it from such that light passes straight through?_ I believe because photons have momentum and the total momentum of the event must be conserved… ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [October 24, 2001, 9:22pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/12 "2001-10-24T21:22:36Z")
> [@](#): > > Interesting…I went to Glenbrook South high school. Do you have something to do with that school or is it just a weird coincidence (from my perspective) that you happened to post a link to there? Purely coincidental, I don’t even remember where I got the link. I don’t even know where Glenbrook is, although it sounds like one of those wimpy California names 🙂 > [@](#): > > Why does an atom re-emit an absorbed photon opposite of the side the photon hit it from such that light passes straight through? Well, Fermat’s principle of least time (least action) says that when light passes from one medium to another it will always take the path of least time. Electromagnetics says that incident light wave will polarize and magnetize the molecules of the medium and, amazingly, the resulting oscillating dipoles will create their own fields which combine with the original fields to create a single wave with the same frequency but different speed. (index of refraction) Quantum Electrodynamics says that the complex phases of the photons cancel out except for the paths of least time. (Feynman’s Sum of Histories path integrals.) This is probably closest to an explanation to what you asked, but in reality no one really knows what a photon or an electron is…. much less what is truly happening. This stuff really sucks. ---
**Author:** ![g8rguy](https://avatars.discourse-cdn.com/v4/letter/g/7cd45c/32.png) [@g8rguy](https://boards.straightdope.com/u/g8rguy)\ **Post date:** [October 24, 2001, 9:48pm UTC](https://boards.straightdope.com/t/speed-of-light-in-water-and-vacuum/89335/13 "2001-10-24T21:48:24Z")
> [@](#): > > \*Originally posted by Whack-a-Mole \* > Why does an atom re-emit an absorbed photon opposite of the side the photon hit it from such that light passes straight through? Ok, I guess light is bent (which you can feel free to explain as well) but I would guess that light would be re-emitted all over the place in random directions. And the answer to the question, **Whack-a-Mole** , is that it **doesn’t** emit a photon on the opposite side, but (as you correctly guessed) in a random direction. However, there’s an **awful** lot of atoms around, and on average, after being scattered a bunch of times, and invoking conservation of momentum, the photon usually comes out the other side. It doesn’t have to, though. After all, you can use a mirror, no? Really, scattering of this sort is much more easily described classically, but on the quantum mechanical level, it’s not as if there’s something that says that the photon momentum must be separately conserved, only the momentum of the entire system.