We
have seen earlier that the total electric and magnetic fields inside a
medium are described in terms of a permittivity e and a magnetic
permeability m (these describe the factors  by which the total fields differ
from the external fields) These replace e0 and m0 in the description to
electric and magnetic fields in Maxwell’s equations with the result that in
a material medium of permittivity e and magnetic permeability m, the
velocity of light becomes,
1
v
µε
=
(8 11)
Thus, the velocity of light depends on electric and magnetic properties of
the medium We shall see in the next chapter that the refractive index of
one medium with respect to the other is equal to the ratio of velocities of
light in the two media