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.

**Further Reading**