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An interesting perspective on the 'speed of light in a vacuum', (which is constant), and the speed of light in normal space, presents itself in the observation of pulsar light.
Ok, so pulsars, emit light pulses simultaneously over a wide range of frequencies (colours of light). But when these pulses arrive at Earth, the higher frequency components arrive before the lower frequencies. This 'dispersion' amongst the different received frequencies is theorised to be due to the ionised interstellar medium. (Space ain't a vacuum, eh ?)
So from the difference in arrival times, the number of electrons present along the path can be calculated (in electrons/square metre).
That's one way they can work out how (un)empty space is (in terms of normal matter). By looking at a whole heap of pulsars, they can combine the data to construct a model of how dense the interstellar matter is, along difference lines of sight.
Pretty cool, if you ask me.
(Just thought I'd share this fascinating snippet … )
Cheers
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