  Weyl fermions play a major role in quantum field theory but have been quite
elusive as fundamental particles. Materials based on quasi two-dimensional
bismuth layers were recently designed and provide an arena for the study of the
interplay between anisotropic Dirac fermions, magnetism and structural changes,
allowing the formation of Weyl fermions in condensed matter. Here, we perform
an optical investigation of YbMnBi$_2$, a representative type II Weyl
semimetal, and contrast its excitation spectrum with the optical response of
the more conventional semimetal EuMnBi$_2$. Our comparative study allows us
disentangling the optical fingerprints of type II Weyl fermions, but also
challenge the present theoretical understanding of their electrodynamic
response.
