The Speed of Light

The speed of light in vacuum has been measured to be 300 million meters per second. This is the fastest that anything has been observed to move. At that speed it would take light a mere one tenth of a second to circumvent the globe. When light enters any material, it slows down. The amount that it slows down depends on the nature of the material. The more dense the material, the slower the speed light. For example, in water, light is approximately 30% slower than in vacuum, while in glass it is 50% slower. In diamand, one of of the most dense material known, light travels at less than 150 million meters per second. Remarkably, in the past year research scientists at MIT have been able to make microscopic quantities of a very strange, and very dense state of matter in which the speed of light is (theoretically) calculated to be just a few meters per second. This slowing down of light plays an important role in how we interpret refraction, which we discuss next.