Originally Posted by wogoga
Light in a vacuum is a disturbance in the electromagnetic field with no electric charge or electric current. So a photon in a vacuum travels with the speed of a disturbance with an electromagnetic field. The constant c is a constant that characterizes disturbances in the electric and magnetic field independent of any electric charge.
If the light wave or photon particles scatters from anything containing electric charge, there is a phase shift corresponding to a time delay. You can imagine an electromagnetic wave being scattered by either a free electron or an entire atom. The electric and magnetic field are pushing the electric charges in the electron or atom. The moving electric charges 'push back' by generating another disturbance that partially cancels the original disturbance in the vacuum.
The electromagnetic wave is 'slowed down' because it is pushing the electric charges each of which have a rest mass greater than zero. A better heuristic picture may be that the photon is absorbed by the electric charges, only to be remitted after a certain time delay. In any case, the scattered wave/particle has a time delay because of the participation of electric current. The light in a vacuum has an electric field component and a magnetic field component ONLY.
It the atoms are close together in a body, then the material in the body is called a medium. Light passed through the medium is not a disturbance in a pure electromagnetic field. The light has an electric field component, a magnetic field component and an electric current component. Thus, it is not a pure electromagnetic wave. The time delay in individual atoms adds up to a reduction in the effective speed of light.
The quantum of energy in a medium may be called a photon, but it is different from the quantum of energy in a vacuum. The photon in the medium is carrying baggage. The baggage is electric current. The photon in a medium consists of an electric field, a magnetic field, and an electric current. There is a property of the medium called the complex dielectric function that has no counterpart in the vacuum. The complex dielectric function characterizes the electric current.
So the photon in a medium is really a photon from the vacuum which has become 'dressed' with an electric current. It may still be called light but it no longer is pure electromagnetic radiation.