  Intermetallic narrow-gap semiconductors have been intensively explored due to
their large thermoelectric power at low temperatures and a possible role of
strong electronic correlations in their unusual thermodynamic and transport
properties. Here we study the optical spectra and vibrational properties of
$\mathrm{FeGa_3}$ single crystal. The optical conductivity indicates that
$\mathrm{FeGa_3}$ has a direct band gap of $\sim 0.7$\,eV, consistent with
density functional theory (DFT) calculations. Most importantly, we find a
substantial spectral weight also below 0.4~eV, which is the energy of the
indirect (charge) gap found in resistivity measurements and ab initio
calculations. We find that the spectral weight below the gap decreases with
increasing temperature, which indicates that it originates from the impurity
states and not from the electronic correlations. Interestingly, we did not find
any signatures of the impurity states in vibrational spectra. The infrared and
Raman vibrational lines are narrow and weakly temperature dependent. The
vibrational frequencies are in excellent agreement with our DFT calculations,
implying a modest role of electronic correlations. Narrow M\" ossbauer spectral
lines also indicate high crystallinity of the sample.
