  We formulate a general framework for describing the electromagnetic
properties of spacetime. These properties are encoded in the `constitutive
tensor of the vacuum', a quantity analogous to that used in the description of
material media. We give a generally covariant derivation of the Fresnel
equation describing the local properties of the propagation of electromagnetic
waves for the case of the most general possible linear constitutive tensor. We
also study the particular case in which a light cone structure is induced and
the circumstances under which such a structure emerges. In particular, we will
study the relationship between the dual operators defined by the constitutive
tensor under certain conditions and the existence of a conformal metric.
Closure and symmetry of the constitutive tensor will be found as conditions
which ensure the existence of a conformal metric. We will also see how the
metric components can be explicitly deduced from the constitutive tensor if
these two conditions are met. Finally, we will apply the same method to explore
the consequences of relaxing the condition of symmetry and how this affects the
emergence of the light cone.
