For a linear medium, the displacement vector and (where is the permittivity of the medium).

The equation 1. 4 results into the wave propagation equation given by

............................(1.5)

From equation 1.5, the velocity of propagation of em wave is given by

....................(1.6)

If the wave is propagating in vacumm then the free space permitivity and permeability are given by =8.85X10^-12s² .C²/m² Kg and =4X10^-7 m.kg/C² then the speed of light in free space is 3X10 8 m/sec, this normally denoted by c and not v.