 The susceptibility is defined as the constant of proportionality (which may be a tensor) relating an electric field to the induced dielectric polarization density such that

where is the electric permittivity of free space.The susceptibility of a medium is related to its relative permittivity by

So in the case of a vacuum,

The susceptibility is also related to the polarizability of individual particles in the medium by the Clausius-Mossotti relation.The electric displacement is related to the polarization density by

The permittivity and permeability of a medium together determine the phase velocity of electromagnetic radiation through that medium