  Accurate and powerful computational methods developed by the author for the
wave scattering by black holes allow to obtain the highly non trivial total
absorption spectrum of the Black Hole. As well as partial wave phase shifts and
cross sections (elastic and inelastic), the angular distribution of absorbed
and scattered waves, and the Hawking emission rates. The exact total absorption
spectrum of waves by the Black Hole has as a function of frequency a remarkable
oscillatory behaviour characteristic of a diffraction pattern. This is an
unique distinctive feature of the black hole absorption, and due to its r = 0
singularity. Ordinary absorptive bodies and optical models do not present these
features. The unitarity optical theorem is generalized to the Black Hole case
explicitly showing that absorption ocurrs only at the origin. All these results
allow to understand and reproduce the Black Hole absorption spectrum in terms
of Fresnel-Kirchoff diffraction theory: by the interference of the absorbed
rays arriving at the origin through different optical paths. These fundamental
features of the Black Hole Absorption will be present for generic higher
dimensional Black Hole backgrounds, and whatever the low energy effective
theory they arise from. In recent and increasing litterature devoted to compute
absorption cross sections (grey body factors) of black holes (whatever
ordinary, stringy, D-braned), the fundamental remarkable features of the Black
Hole Absorption spectrum are overlooked.
