Not silly at all. Indeed the speed of light varies in different mediums with different densities. Why we see a pencil appear to bend when lowered into a glass of water.
When we talk about c (speed of light), we are referring to the speed of light in a vacuum.
See:
http://library.thinkquest.org/27066/lightrays/nlrefraction.html
And:
http://en.wikipedia.org/wiki/Dispersion_(optics)
And finally:
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/speed_of_light.html
Glad my magnetron is not in a vacuum. More chocolate!