Is Light's Phase Velocity Always C?

[Muon321]

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

[elfmotat]

Phase velocity is [math]v_{p}=E/p[/math], and group velocity is [math]v_{g}=\partial E/\partial p[/math]. For light, [math]E=pc[/math], so [math]v_p = v_g = c[/math].