  Accurate and powerful analytic and computational methods developped by the
author allow to obtain the highly non trivial total absorption spectrum of the
Black Hole, as well as 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
presents as a function of frequency a remarkable oscillatory behaviour
characteristic of a diffraction pattern. It oscillates around its optical
geometric limit (27/4) pi (r_s)^2 with decreasing amplitude and almost constant
period. 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 Hamiltonian describing the wave-black hole
interaction is non hermitian (despite being real) due to its singularity at the
origin (r=0). The unitarity optical theorem of scattering theory is generalized
to the black hole case explicitely showing that absorption takes place only at
the origin (r = 0). All these results allow to understand and reproduce the
Black Hole absorption spectrum in terms of Fresnel-Kirchoff diffraction theory.
These fundamental features 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 on 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.
