I've been looking all over the web for this stuff and I simply can't find anything that gives a proper explanation. What determines the speed of say, radio waves, in a medium? I know electromagnetic waves travel at the speed of light and as such I guess I can use the refractive index to figure it out, but what of materials where a refractive index isn't at hand? A few examples question to get my meaning across a bit better: How fast does radio waves propagate in fresh water? Do they propagate at the same speed in sea water? What if I increase the salinity of the sea water, does that change the speed? What about something like a dense vs a not so dense plastic? Can it be assumed that a more dense material also slows down the radio waves more or is it something completely different that affects the speed?