It travels fastest through a vacuum where there are no molecules of any kind to get in the way and slow it down. When light passes through one material to the next, it undergoes refraction. Every material has something called an index of refraction. This number is found by dividing the speed of light through a vacuum, which is 3 million meters per second, by the speed of light through that particular material. The index of refraction will always be greater than one because light always travels slower when it isn't in a vacuum. Because light travels differently through different materials, we can see the water level in a clear glass even though the water, air, and the glass are all clear materials. Light bends at a different angle when it hits each material.