Fermilab

Everybody agrees, that when light propagates through a medium, the light and the medium most likely will interact. Let us see how this interaction affects the speed of light.

Remember how we got the parameter c: from Maxwell's equations; we derived the wave equations and then read out the speed of light.When we deal with electromagnetic waves in matter, we have to modify Maxwell's equations. Remarkably, for our purposes,the same two parameters, permittivity and permeability suffice.

Every material has its own permittivity and permeability, just as a vacuum does.The numerical values come from precise measurements.

After measuring the permittivities and permeabilities for the material of interest, the speed of light can be calculated according to the formula

Notice that the permeability and permittivity values in these equations are those measured for various materials and for a vacuum.

IT CAN BE PROVEN THAT THE SPEED OF LIGHT IN MATERIALS IS ALWAYS LESS THAN THE SPEED OF LIGHT IN A VACUUM!

On the following pages, we discuss the connection between the speed of light and Einstein's theory of special relativity.