# Physical origin of different phase velocities.

When monochromatic light propagates through a material other than a vacuum, we observe that its phase velocity has moved away from c. By relativity, we know that massless particles like the photon must move at the c however the pragmatics of classical optics ask of us to overlook this fact in favor of the practical model in which the speed of light in material can have variety. This are technically contradictory models and something has to give, and I'm 99% certain its classical optics.

So when it is said that the speed of light in a medium is some phase velocity $$v_p$$ what does this physically mean, and why the observed deviation from c?

DrDu