The optical properties of the superconducting K \(_{0.8}\) Fe \(_{1.7}\) (Se \(_{0.73}\) S \(_{0.27}\) ) \(_2\) single crystals with a critical temperature \(T_c\approx 26\) K have been measured in the ab plane in a wide frequency range using both infrared Fourier-transform spectroscopy and spectroscopic ellipsometry at temperatures of 4–300 K. The normal-state reflectance of K \(_{0.8}\) Fe \(_{1.7}\) (Se \(_{0.73}\) S \(_{0.27}\) ) \(_2\) is analyzed using a Drude-Lorentz model with one Drude component. The temperature dependences of the plasma frequency, optical conductivity, scattering rate, and dc resistivity of the Drude contribution in the normal state are presented. In the superconducting state, we observe a signature of the superconducting gap opening at \(2\Delta (0)=11.8\) meV. An abrupt decrease in the low-frequency dielectric permittivity \(\varepsilon _1(\omega )\) at \(T < T_c\) also evidences the formation of the superconducting condensate. The superconducting plasma frequency \(\omega _{pl,s} = (213\pm 5)\) cm \(^{-1}\) and the magnetic penetration depth \(\lambda =(7.5\pm 0.2)\) \(\mu\) m are determined.
