The index of refraction of materials is caused by differences of the speed of light in those materials. However, according to the theory of relativity, the spee

The index of refraction of materials is caused by differences of the speed of light in those materials. However, according to the theory of relativity, the speed of light in a _vacuum_ is constant in all reference frames. There is a neat web page where you can see the speed of light in different materials: https://micro.magnet.fsu.edu/primer/java/speedoflight/index.html#:~:text=Light%20travels%20at%20approximately%20300%2C000,(refractive%20index%20of%201.5) https://micro.magnet.fsu.edu/primer/java/speedoflight/index.....