Roy Lewallen wrote:

As others have pointed out, it's risky to treat c as a variable or
medium-dependent speed.

Roy -

The convenient thing about using medium dependent c in that equation
is that we can use things such as index of refraction or velocity
factor to convert from vacuum 'c' to 'c' in another medium. The fact
that it makes the results of the calculation more accurate tends to
mitigate any risk that might be encumbered when using it.

To require that only vacuum c be used in the equation to me seems
overly authoritarian. I wonder how you feel about the speed of sound? :-)

73, Jim AC6XG


That letter is nearly always used to designate
the speed of light (or any EM plane wave) in a vacuum. Using that
nearly-universal definition, the speed of an EM wave in any other medium
is VF * c where VF is the "velocity factor". It's important to realize
that while there's a single value for the speed of all EM waves in a
vacuum (c), this isn't true in many other media. In many media, the
speed of the wave depends on its frequency, a phenomenon called
"dispersion". So in many media there's no universal EM velocity
equivalent to c, but rather a frequency-dependent velocity factor.

In environments where the field is confined such as a waveguide, the
velocity can also depend on the mode, that is the orientation of the
fields. So there's not even a single value for each frequency. And this
can be true even if the waveguide is filled with a vacuum.

Roy Lewallen, W7EL