Light travels at a speed of 186,282 miles per second, in a vacuum. This is described by the Theory of Relativity equation E=mc2. The c in the equation relates space with time. In different materials, the speed of light slows down to less than c. That is because light refracts when it encounters different mediums. The refractive index or n shows the relationship between c and the speed v when light travels through a material. The equation is n=c/v. In water, the refractive index is 1.3 and in glass, it is 1.5. Therefore, light travels faster through water, than it travels through glass.
light travels fastest in vaccume and the least in water and in glass because the molecules of water and glass strikes the photons of the light. there is no air in space it is empty (vaccume)so, light travels fastest in vaccume. 299,792,458 metre/second.
speed of light decreases with an increase in density of the medium
density of glass>density of water
so light travels fastter in water