  The optical activity tensor (OAT) is explicitly derived. It is shown that to
evaluate a large number of effects related to optical activity of some
atomic/molecular system at arbitrary frequency $\omega$ of the incident light,
one needs to know only four optical activity tensors which have twelve
irreducible (tensor) components. An additional amplification factor contains
one $3 \times 3$ tensor of light scattering with three irreducible components.
The explicit dependence of all irreducible components of OAT upon $\omega$ and
some molecular parameters is derived and discussed. We apply OAT to explain the
dispersion of optical rotation in dilute solutions of organic molecules. This
study opens a new avenue in application of methods of modern Quantum
Electrodynamics to the optical activity.
