  Among different topological and related phases of condensed matter, nodal
semimetals occupy a special place - the electronic band topology in these
materials is related to three-dimensional bulk, rather than to surface, states.
A great variety of different realizations of electronic band crossings (the
nodes) leads to a plethora of different electronic properties, ranging from the
chiral anomaly to solid-state realizations of a black-hole horizon. The
different nodal phases have similar low-energy band structure and quasiparticle
dynamics, which both can be accessed experimentally by a number of methods.
Optical measurements with their large penetration depth and high energy
resolution are ideally suited as such a bulk probe; especially at low energies
where other spectroscopic methods often lack the required resolution. In this
contribution, we review recent optical-conductivity studies of different nodal
semimetals, discuss possible limitations of such measurements, and provide a
comparison between the experimental results, simple theoretical models, and
band-structure-based calculations.
