We report the full complex dielectric function of high-purity Ba0.68K0.32Fe2As2 single crystals with Tc = 38.5 K determined by wide-band spectroscopic ellipsometry at temperatures 10 ≤ T ≤ 300 K. We discuss the microscopic origin of superconductivity-induced infrared optical anomalies in the framework of a multiband Eliashberg theory with two distinct superconducting gap energies 2∆A ≈ 6 kBTc and 2∆B ≈ 2.2 kBTc. The observed unusual suppression of the optical conductivity in the superconducting state at energies up to 14 kBTc can be ascribed to spin-fluctuation–assisted processes in the clean limit of the strong-coupling regime. PACS numbers: 74.25.Gz,74.70.Xa,74.20.Mn,78.30.-j The discovery of iron-based superconductors [1] has gen-erated significant experimental and theoretical effort to un-ravel the mechanism of high-temperature superconductivity in these compounds. This effort has yielded a comprehen-sive experimental description of the electronic structure at the Fermi level, which includes multiple Fermi surface sheets in a good agreement with density functional calculations [2, 3]. Partial nesting between at least two of these sheets leads to

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