# The speed of light and it's variation through different media? [duplicate]

Does the speed of light vary?

Special Relativity says it's constant in all Inertial frames; but what if it was to travel through different media?

The speed of light does vary when it changes (travels through) different media:

Here is why:

Firstly, you need to acknowledge that Electromagnetic waves propagate through space as a wave but interact with matter as a particle (a photon). This is known as wave-particle duality and requires some basic knowledge of Quantum Mechanics.

Consider a light wave that enters matter, the light wave's electric field causes charged particles in the matter to accelerate back and forth. This is because an electric field exerts forces on charged particles. The light wave gives up some of its energy to these charged particles and is partially absorbed in the process. However, the charged particles don't retain the light's energy very long. They are accelerating and accelerating charged particles emit electromagnetic waves. In fact, they re-emit the very same light wave that they absorbed moments earlier. Overall, the light wave is partially absorbed and then re-emitted by each electrically charged particle it encounters, so that the light continues on its way as though nothing had happened.

However, something has happened; the light wave has been delayed ever so slightly. This absorption and re-emission process holds the light wave back so that it travels at less than its full speed. If the charged particles in the matter are few and far between, this slowing effect is almost insignificant. But in dense materials such as glass or diamond, the light wave can be slowed substantially.

Taking the extreme case where there is no media present whatsoever (a vacuum) then the speed of the electromagnetic waves is optimal, and equal to $c$.