  Optical spectroscopy was used to study the electrodynamics and hidden
transport properties of a BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ thin superconducting
film. We analyzed the normal state data using a Drude-Lorentz model with two
Drude components: one narrow ($D_1$) and another broad one ($D_2$). In the
superconducting state, two gaps with $2\Delta _{0}^{(2)}/k_BT_c=1.9$--2.0 and
$2\Delta _{0}^{(1)}/k_BT_c=4.0$--4.3 are formed from the narrow component $D_1$
while the broad component $D_2$ remains ungapped. The calculated total DC
resistivity of the film and the low-temperature scattering rate for the narrow
Drude component show a hidden Fermi-liquid behavior. The change of total
electron-boson coupling ($\lambda_{tot}$) and representative energy
($\Omega_{0}$) in the normal state with respect to the superconducting state is
typical of other iron-based materials as well as high-temperature
superconducting (HTSC) cuprates.
