  A uniaxial strain applied to graphene-like materials moves the Dirac nodes
along the boundary of the Brillouin zone. An extreme case is the merging of the
Dirac node positions to a single degenerate spectral node which gives rise to a
new topological phase. Then isotropic Dirac nodes are replaced by a node with a
linear behavior in one and a parabolic behavior in the other direction. This
anisotropy influences substantially the optical properties. We propose a method
to determine characteristic spectral and transport properties in black
phosphorus layers which were recently studied by several groups with
angle-resolved photoemission spectroscopy, and discuss how the transmittance,
the reflectance and the optical absorption of this material can be tuned. In
particular, we demonstrate that the transmittance of linearly polarized
incident light varies from nearly 0\% to almost 100\% in the microwave and
far-infrared regime.
