Technically the light itself does not slow down in water, it's the combined electromagnetic wave effect caused by the material:
...
...[video snipped away for quote]
I've seen that guy(in the video) in other presentations, he is rather good.
Whether how light slow down or not is an interesting point to bring up, but how it slow down really doesn't alter the fact that it did slow down. That however brings up a couple of cans of worms that is very interesting. I was debating whether I should bring those complication up - or not, but with that video, that gives me a reason to get into it. (I actually had passion about the first can of worm once upon a time -- way before SI's definition of Time quantity became real)
First can of wormsAt the time Eisenstein came up with time dilation and speed of light being a important factor, Time was not we defined as Time today in the SI unit -- that is why I included the SI definition of time in my first reply.
In Eisenstein's days, EM wave (light) propagation was understood to be electric field inducing a magnetic field which in-turn induce an electric field which in-turn induce a magnetic field, so on, so the EM wave propagates. Fine, we accept that. So, how fast one field induce the next how that next field induce another next, so on, takes time, so there is a speed limit, fine, we accept that. And with that light/EM wave and time is rather closely linked as other theories developed, and in particular, how Eisenstein think of time since relativity is the one that changed how we view time.
When SI-Unit for Time was introduced, whether that definition "the unperturbed ground-state hyperfine transition frequency of the caesium 133 atom" preserved fully (ie:
quantitative vs qualitative) the nature of time (and its impact on other accepted theories) is an interesting discussion that some still consider an open one after SI unit was published. Whether that was a finished discussion, or was an abandoned discussion, is unknown to me. So, not wanting to touch the mutability of the definition of time, I just inserted the definition, and added a jab about some experimental physics there would interesting.
Second can of wormsThe other complication that I hand-waved over is velocity. Velocity alone doesn't cause time dilation, acceleration does. We write our stuff talking about velocity, (common writing about twins paradox where one twin travels and one twin doesn't travel for example), we
pretend:
under the cover that without acceleration you don't get velocity. But in fact, it is the acceleration that one twin must do (and the non-traveling twin doesn't) that caused the time difference.
Eating up the wormsThe pretending is because the deeper one digs, the more difficult it is to explain the dirt that one digs up. So, staying with generally accepted principal and brushing over the deeper complications (
while staying true to observed result) is the way around having a 100 page explanation that no one wants to read anyway. (Or trying to explain the Twin Paradox's explanation is actually wrong, it is not velocity, it is acceleration... One would be arguing with a thousand people a thousand times over.)
So as long as the "light travel slower in water" matches the observed result, that's fine. But digging deeper will always uncover more interesting facts. Some of the facts may even be Nobel Prize material, who knows.