  Topological semimetals are predicted to exhibit unconventional
electrodynamics, but a central experimental challenge is singling out the
contributions from the topological bands. TaAs is the prototypical example,
where 24 Weyl points and 8 trivial Fermi surfaces make the interpretation of
any experiment in terms of band topology ambiguous. We report magneto-infrared
reflection spectroscopy measurements on TaAs. We observed sharp inter-Landau
level transitions from a single pocket of Weyl Fermions in magnetic fields as
low as 0.4 tesla. We determine the W2 Weyl point to be 8.3 meV below the Fermi
energy, corresponding to a quantum limit - the field required to reach the
lowest LL - of 0.8 Tesla - unprecedentedly low for Weyl Fermions. LL
spectroscopy allows us to isolate these Weyl Fermions from all other carriers
in TaAs and our result provides a new way for directly exploring the more
exotic quantum phenomena in Weyl semimetals, such as the chiral anomaly.
