They defined the meter as the distance light traveled in vacuum for 1299792458 of a second such that the speed of light in a vacuum is drum roll 299792458 ms or 299792458 kms. This value is the speed of light in a vacuum. Glass n 150.
The speed of light in vacuum is c 299792458 x 108ms.
The refractive index of a material is a measure of the change in the speed of light as it passes from a vacuum or air as an approximation into the material. The National Bureau of Standards in Boulder Colorado used helium-neon lasers and meticulously accurate cesium clocks to measure the speed of light. When light enters a substance such as glass its speed is reduced based on how it hits the atoms in the material. The reason is that it scatters off of the molecules in the material and is slowed down.