  By combining the Dirac equation of relativistic quantum mechanics with the
Bogoliubov-De Gennes equation of superconductivity we investigate the
electron-hole conversion at a normal-metal--superconductor interface in
graphene. We find that the Andreev reflection of Dirac fermions has several
unusual features: 1) The electron and hole occupy different valleys of the band
structure; 2) At normal incidence the electron-hole conversion happens with
unit efficiency in spite of the large mismatch in Fermi wave lengths at the two
sides of the interface; and, most fundamentally: 3) Away from normal incidence
the reflection angle may be the same as the angle of incidence
(retro-reflection) or it may be inverted (specular reflection). Specular
Andreev reflection dominates in weakly doped graphene, when the Fermi wave
length in the normal region is large compared to the superconducting coherence
length. We find that the transition from retro-reflection to specular
reflection with decreasing doping is associated with an inversion of the
voltage dependence of the subgap conductance of the interface.
