AbstractWe measured the optical reflectivity R(ω) for an underdoped (Ca_0.935La_0.065)₁₀(Pt₃As₈)(Fe₂As₂)₅ single crystal and obtained the optical conductivity ${σ }_{1}(ω )$ σ 1 ( ω ) using the K-K transformation. The normal state ${σ }_{1}(ω )$ σ 1 ( ω ) at 30 K is well fitted by a Drude-Lorentz model with two Drude components (ω_p1 = 1446 cm⁻¹ and ω_p2 = 6322 cm⁻¹) and seven Lorentz components. Relative reflectometry was used to accurately determine the temperature dependence of the superconducting gap at various temperatures below T_c. The results clearly show the opening of a superconducting gap with a weaker second gap structure; the magnitudes for the gaps are estimated from the generalized Mattis-Bardeen model to be Δ₁ = 30 and Δ₂ = 50 cm⁻¹, respectively, at T = 8 K, which both decrease with increasing temperature. The temperature dependence of the gaps was not consistent with one-band BCS theory but was well described by a two-band (hence, two gap) BCS model with interband interactions. The temperature dependence of the superfluid density is flat at low temperatures, indicating an s-wave full-gap superconducting state.