The Speed of Light

Does the speed of light depend on the medium?

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.


This is a web preview of the "The Handy Physics Answer Book" app. Many features only work on your mobile device. If you like what you see, we hope you will consider buying. Get the App