  The luminous electromagnetic emission from distant active galactic nuclei
(AGNs) including quasars is believed to be powered by accretion onto
super-massive black holes (SMBHs). In the standard unification model for AGNs a
dusty torus covers a significant portion of the viewing angles to the accretion
disk and the BH. The system is classified as a type-I AGN if the accretion disk
is viewed through the opening part; otherwise it is called a type-II AGN.
Therefore the ratio of type-II to type-I AGNs serves as a sensitive probe to
the unification model. A surprising discovery made from several large sky
coverage and/or deep AGN surveys has found a significant anti-correlation
between the type-II fraction and the observed X-ray luminosity between 2-10
keV. This suggests two different luminosity functions for the two types of
AGNs, thus challenging the AGN unification model. However this observed
anti-correlation is a natural consequence of the AGN unification model with
only one intrinsic luminosity function if the inclination angle effects of the
X-ray emitting accretion disk are taken into account. Thus the AGN unification
model survived another critical test.
