They differ from all the other waves we have
studied so far, in respect that no material medium is involved in the
vibrations of the electric and magnetic fields But what if a material medium is actually there We know that light,
an electromagnetic wave, does propagate through glass, for example 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)