Light travels well through air or glass, but poorly through substances like wood or metal.
Light moves at different speeds in different media. It travels fastest through a vacuum, and slower through a medium like Earth s atmosphere. As a result, where there is a boundary between different media, the direction of a light beam can be altered. We can measure this change, called the index of refraction. To communicate using a fiber optic cable, a light source produces pluses of light that travel through the cable.