  Using polarized optical and magneto-optical spectroscopy, we have
demonstrated universal aspects of electrodynamics associated with Dirac
nodal-lines. We investigated anisotropic electrodynamics of NbAs$_2$ where the
spin-orbit interaction triggers energy gaps along the nodal-lines, which
manifest as sharp steps in the optical conductivity spectra. We show
experimentally and theoretically that shifted 2D Dirac nodal-lines feature
linear scaling $\sigma_1 (\omega)\sim\omega$, similar to 3D nodal-points.
Massive Dirac nature of the nodal-lines are confirmed by magneto-optical data,
which may also be indicative of theoretically predicted surface states. Optical
data also offer a natural explanation for the giant magneto-resistance in
NbAs$_2$.
