1.1 The propagation of light  (Page 2/6)

The French physicist Jean Bernard Léon Foucault (1819–1868) modified Fizeau’s apparatus by replacing the toothed wheel with a rotating mirror. In 1862, he measured the speed of light to be $2.98\times {10}^8\text{m/s},$ which is within 0.6% of the presently accepted value. Albert Michelson (1852–1931) also used Foucault’s method on several occasions to measure the speed of light. His first experiments were performed in 1878; by 1926, he had refined the technique so well that he found c to be $(2.99796\pm 4)\times {10}^8\text{m/s}.$

Today, the speed of light is known to great precision. In fact, the speed of light in a vacuum c is so important that it is accepted as one of the basic physical quantities and has the value

where the approximate value of $3.00\times {10}^8\text{m/s}$ is used whenever three-digit accuracy is sufficient.

Speed of light in matter

The speed of light through matter is less than it is in a vacuum, because light interacts with atoms in a material. The speed of light depends strongly on the type of material, since its interaction varies with different atoms, crystal lattices, and other substructures. We can define a constant of a material that describes the speed of light in it, called the index of refraction     n :

where v is the observed speed of light in the material.

Since the speed of light is always less than c in matter and equals c only in a vacuum, the index of refraction is always greater than or equal to one; that is, $n\ge 1$ . [link] gives the indices of refraction for some representative substances. The values are listed for a particular wavelength of light, because they vary slightly with wavelength. (This can have important effects, such as colors separated by a prism, as we will see in Dispersion .) Note that for gases, n is close to 1.0. This seems reasonable, since atoms in gases are widely separated, and light travels at c in the vacuum between atoms. It is common to take $n=1$ for gases unless great precision is needed. Although the speed of light v in a medium varies considerably from its value c in a vacuum, it is still a large speed.