  Using a novel approach to calculate optical properties of strongly correlated
systems, we address the old question of the physical origin of the alpha to
gamma transition in Ce. We find that the Kondo collapse model describes the
optical data better than the Mott transition picture. Our results compare well
with existing thin--film experiments. We predict the full temperature
dependence of the optical spectra and find the development of a pseudogap
around 0.6 eV in the vicinity of the alpha to gamma phase transition.
