3
votes
3answers
149 views

Why is the phase velocity used in the definition of the refractive index?

I'm aware of the so-called group index but why is the phase velocity used in the standard definition of the index of refraction? What advantage does this offer?
2
votes
3answers
491 views

Two different mediums can have equal index of refraction?

In other words: given a certain medium, is light's phase velocity unique?
3
votes
1answer
271 views

Fermat principle: which index of refraction?

I am somewhat puzzled by a common formulation of the Fermat principle (light travel time), because it contains index of refraction related to phase velocity while light travel time through a slab of ...