Here, we report on a magneto-optical study of two distinct systems hosting massless fermionstwo-dimensional graphene and three-dimensional HgCdTe tuned to the zero band gap condition at the point of the semiconductor-to-semimetal topological transition. Both materials exhibit, in the quantum regime, a fairly rich magneto-optical response, which is composed from a series of intraand interband inter-Landau level resonances with for massless fermions typical ffiffiffi B p dependence. The impact of the system's dimensionality and of the strength of the spin-orbit interaction on the optical response is also discussed. V C 2015 AIP Publishing LLC.