Light is an electromagnetic wave. Physical waves – sound, waves on strings etc – involve the vibration of matter. Electromagnetic waves involve the vibration of magnetic fields and electric fields. They create each other – a changing electric field produces a magnetic field, and a changing magnetic field produces a magnetic field with electric and magnetic fields at right angles to each other. They couple together in a self sustaining way, enabling the wave to propagate over the vast expanses of space.
The laws of physics which allow us to model the electromagnetic waves are called Maxwells' equations. The speed of the waves
is predicted in terms of magnetic and electric constants:
where
= electric susceptibility of free space
and
= magnetic permeability of free space.
There is an important consequence from the expression of the speed of light in terms of physical constant: the speed of light is constant, so does not obey the Galilean transformation. It is independent of the speed of the source, observer howsoever they are in relative motion to each other. This lead in the late nineteenth and early twentieth centuries to the superceding of Newtonian mechanics first by the special theory of relativity, then by the general theory of relativity.