The speed at which light is transmitted that travels through a material that is transparent like the air and glass is slower than. A ratio that is the ratio of c with speed v at which light travels across the material is known as”the refractive index” of the medium (n = c/v). For instance in the case of visible light, the refractive index of glass is an approximate value of 1.5 (which means that light moves across the surface at a rate of approximately 200000 km/s. The refractive index for the air that is visible to light has a value of 10003 which means that the speed of air light will be around 90 km/s faster than the speed in air.

The speed of light travels at the vacuum at its highest, and dependent on the medium glass, water, or diamonds will slow down.