Patterns of electric or magnetic Polarization of a medium which are usually caused by Light waves.

When an optical wave propagates in a medium the associated electromagnetic field generates patterns of electric and magnetic polarization of the medium which propagate together with the generating optical wave. In most cases of propagation in Transparent media the electric polarization wave has a much stronger effect than the corresponding magnetic wave, because the electric susceptibility is much higher than the magnetic one. The interaction of the electric polarization wave with the Electromagnetic Wave reduces the Phase Velocity of the combined phenomenon below the vacuum velocity of light.