I refer to the velocity of propagation of an EM wave in a medium. I have been wondering about what plays the role, in this context, of inertia and elasticity. Here the formula has nothing in the numerator and the denominator is the product of the electric permittivity by the magnetic permeability of the material in question. Initially I thought that permittivity and permeability played the role of inertia, because they occupy the denominator (the higher they are, the lower the velocity) and also because some books speak of “optical density”. But then I heard the contrary opinion and also realized the following: permittivity is the facility of the atoms (or atoms network if you wish) to form dipoles and polarization is a sort of elastic tension; so what happens with this concept is that we are talking about something akin to Young modulus (in a solid) or Bulk modulus (in a fluid), it is only that the latter take the perspective of difficulty of deformation whilst permittivity would be easiness of deformation. This would make sense in terms of dimensions since elasticity modulus is Newtons x m-2, whilst permittivity is Newtons -1 x m-2. However, the dimensions of permittivity also have charge squared. And in any case inertia must be somewhere. So can it be then that permittivity is an empirical concept that mixes the two components: elasticity (or rather its opposite) and a sort of inertia?