[kosta50]

The speed of light is the same for all observers. It's independent of the motion of the light source and of the frame of reference of the observer. So maybe "which time did he use" is irrelevant.

But here is the problem: we know that light doesn't travel at the same speed in different media, such as glass. If it didn't lenses would be worthless.

Snells law or refraction is validated by the fact that light does slow down when it encounters glass. Such encounter causes the wavefront to become "deformed" or "retarded" relative to the portion of the wavefront that is still traveling outside of glass.

This "retardation" or deformation of the wavefront is responsible for causing a plane wave to become spherical with focus at its center. Also, depending on the shape of the surface encountered, it can become aspherical and cause optical aberration at the focus.

A skillful optical engineer will introduce another element, glass or mirror, or even set of elements, into the light path to deform the deformed parts of the wavefront so that that it returns to a spherical front,and form an aberration-free image of an object.

[D-fendr]

I love wave theory, how waves share behaviour whether it’s water or light. Working in optics must be a fascinating occupation.

I don’t think refraction violates the Theory of Relativity. That electromagnetic radiation in a vacuum is the same for all observers is the point, not that it still doesn’t refract. This speed is taken as the constant - a universal constant, and the rest of relativity can be seen as flowing from this. IIRC, this was the insight that led to the development of the theory. If the speed of light (in a vacuum) is a universal constant for all inertial frames, then what must happen to space, time, mass..