  The temperature dependences of the plasma frequency, superfluid density and
London penetration depth were determined from terahertz spectra of conductivity
and dielectric permittivity of BaFe$_{1.91}$Ni$_{0.09}$As$_{2}$ film with
critical temperature $T_c=19.6$~K. Part of experimental data were analyzed
within a simple three-band Eliashberg model where the mechanism of
superconducting coupling is mediated by antiferromagnetic spin fluctuations,
whose characteristic energy $\Omega_{0}$ scales with $T_{c}$ according to the
empirical law $\Omega_{0} = 4.65k_{B}T_{c}$, and with a total electron-boson
coupling strength $\lambda_{tot} = 2.17$.
