Thread: Is Light's Phase Velocity Always C?

If it is not always c, then λf=c would be incorrect, and it should be λf=v_p like matter waves, right?

The photon's velocity is always c. Electromagnetic waves are also used in situations where they are not the eigenfunctions of single photons.

Originally Posted by muon321

If it is not always c, then λf=c would be incorrect, and it should be λf=v_p like matter waves, right?

Speaking classically:

In a vacuum the phase velocity is c. In a medium it may not be. The ratio c/v_p is the refractive index.