  Ballistic electrons in phosphorene $pn$ junctions show optical-like
phenomena. Phosphorene is modeled by a tight-binding Hamiltonian that describes
its electronic structure at low energies, where the electrons behave in the
zigzag direction as massive Dirac fermions and in the orthogonal armchair
direction as Schr\"odinger electrons. Applying the continuum approximation, we
derive the electron optics laws in phosphorene $pn$ junctions, which show very
particular and unusual properties. Due to the anisotropy of the electronic
structure, these laws depend strongly on the orientation of the junction with
respect to the sublattice. Negative and anomalous reflection are observed for
tilted junctions, while the typical specular reflection is found only, if the
junction is parallel to the zigzag or armchair edges. Moreover,
omni-directional total reflection, called anti-super Klein tunneling, is
observed if the junction is parallel to the armchair edge. Applying the
nonequilibrium Green's function method on the tight-binding model, we calculate
numerically the current flow. The good agreement of both approaches confirms
the atypical transport properties, which can be used in nano-devices to
collimate and filter the electron flow, or to switch its direction.
