  A semi-classical description of the intrinsic spin-Hall effect (SHE) is
presented which is relevant for a wide class of systems. A heuristic model for
the SHE is developed, starting with a fully quantum mechanical treatment, from
which we construct an intuitive expression for the spin-Hall current and
conductivity. Our method makes transparent the physical mechanism which drives
the effect, and unifies the SHE across several spintronic and optical systems.
Finally, we propose an analogous effect in bilayer graphene.
