Waves travel at different speeds when traveling in different materials. In the vacuum of space, light travels at 299,792 km/s. When light encounters a denser medium, however, like Earth’s atmosphere, it slows down ever so slightly to 298,895 km/s. Upon striking water it slows down rather dramatically, to 225,408 km/s, three quarters of its original speed. Finally, when light passes through the dense medium of glass, it slows to only 194,670 km/s. The ratio of the speed of light in a vacuum to that in a medium is called the refractive index, represented by the symbol n. The refractive index will be discussed in further detail in the section on lenses.