  Optical response of crystalline solids is to a large extent driven by
excitations that promote electrons among individual bands. This allows one to
apply optical and magneto-optical methods to determine experimentally the
energy band gap - a fundamental property crucial to our understanding of any
solid - with a great precision. Here we show that such conventional methods,
applied with great success to many materials in the past, do not work in
topological Dirac semimetals with a dispersive nodal line. There, the optically
deduced band gap depends on how the magnetic field is oriented with respect to
the crystal axes. Such highly unusual behaviour is explained in terms of
band-gap renormalization driven by Lorentz boosts.
