In Chapter 26, we learned that light travels more slowly in glass than in air. Does this contradict Einstein’s second postulate?
Einstein's second postulate was about light traveling the same speed in all reference frames, only in the case of free space speed of light.the question is about Refraction phenomena of optically dense media it is not the part of relativity. When light travels through a medium like water or glass is quite complex and involves the interaction of light with the material it is traveling through.the light always travels at the speed of light,but consider an material it moves with speed of light until it hits an atom.it is then observed and re-emitted in same direction for that it will take some time.glass is denser material that's why it takes more time.so the speed of light is less in glass medium compare to the air medium. Or In general we know the refractive index of air is 1.000293 and refractive index of glass is (1.52-1.54) depending on the material.then n g n soathe speed of light is more in air than glass.