General Knowledge - Physics - Discussion

The speed of light at which light propagates through glass and air is less than c and because the value of c is less so the refractive index value is also decreases the refractive index is the ratio of c and the velocity of light in material thats why the speed of light is fastest in vacuum.

The speed of light is greater in vaccum as the light waves do not require a material medium to propagate. I think it is vaccum.

It's vacuum which means empty place. Because, of molecular structure. The molecules in glass (solid) place close together and the molecules in water (liquid) place together but with more space comparing to glass. But in vacuum there is lot of space between the molecules also comparing to hydrogen. So, the light can travel at the fastest speed in vacuum.