[lilgreenland]

thanks! When I wrote that line I think I went with the top result of physics.stackexchange, but I'm now convinced that the phase velocity explanation is better.

Here is my updated text:

<p>While individual waves of light always move at c, light seems to move slower in dense media. This is because as the light wave propagates through a medium it produces ripples that <a href="https://www.youtube.com/watch?v=CiHN0ZWE5bk">interfere</a> in a way that slows the group velocity of the light wave.</p>