We investigate the optical conductivity and far-infrared magneto-optical response of ${\mathrm{BaNiS}}_2$, a simple square-lattice semimetal characterized by Dirac nodal lines that disperse exclusively along the out-of-plane direction. With the magnetic field aligned along the nodal line the in-plane Landau level spectra show a nearly $\sqrt{B}$ behavior, the hallmark of a conical-band dispersion with a small spin-orbit coupling gap. The optical conductivity exhibits an unusual temperature-independent isosbestic line, ending at a Van Hove singularity. First-principles calculations unambiguously assign the isosbestic line to transitions across Dirac nodal states. Our work suggests a universal topology of the electronic structure of Dirac nodal lines.

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• Electronic structure
• First-principles calculations
• Landau levels
• Magneto-optical spectra
• Optical conductivity
• Topological materials
• Topological phases of matter
• Van Hove singularity
• Dirac semimetal
• Nodal-line semimetals
• Ab initio calculations
• Optical spectroscopy