  Transition metal monopnictides represent a new class of topological
semimetals with low-energy excitations, namely, Weyl fermions. We report
optical properties across a wide spectral energy range for TaAs, TaP, NbAs and
NbP, calculated within density functional theory. Spectra are found to be
somewhat independent of the anion and the light polarization. Their features
are explained in terms of the upper $s$, $p$, $d$, and $f$ electrons.
Characteristic absorption features are related to the frequency dependence of
the Fresnel reflectivity. While the lower part of the energy loss spectra is
dominated by plasmonic features, the high-energy structures are explained by
interband transitions.
