  We use general symmetry-based arguments to construct an effective model
suitable for studying optical properties of lead-halide perovskites. To build
the model, we identify an atomic-level interaction between electromagnetic
fields and the spin degree of freedom that should be added to a
minimally-coupled $\mathbf{k\cdot p}$ Hamiltonian. As a first application, we
study two basic optical characteristics of the material: the Verdet constant
and the refractive index. Beyond these linear characteristics of the material
the model is suitable for calculating non-linear effects such as the
third-order optical susceptibility. Analysis of this quantity shows that the
geometrical properties of the spin-electric term imply isotropic optical
response of the system, and that optical anisotropy of lead-halide perovskites
is a manifestation of hopping of charge carriers. To illustrate this, we
discuss third-harmonic generation.
