A new p-n interface made of p-type MgO as an optical window to the n-type Ga4Se3S crystals is investigated by means of optical reflectance, transmittance and absorbance in the incident light wavelength (k) range of 200-1100 nm. The reflectivity spectral analysis as a function of angle of incidence for MgO, Ga4Se3S and the Ga4Se3S/MgO layers revealed Brewster angles of 75 degrees, 80 degrees and 70 degrees with the corresponding dielectric constants of 13.93, 32.16 and eMgO 7: 55eGa(4)Se(3)S, respectively. To remove Brewster condition of reflection and obtain maximum absorption, the light must be incident from the MgO side. A novel light absorbability is observed. Namely, for all k < 600 nm, the absorbance is dominated by the Ga4Se3S layer. For larger k values, while the crystal absorbance decreases significantly, the bilayer absorbance increased by four times in the visible range and three times in the IR range of spectrum. In the MgO layer, two distinct sets of band tails of the localized states with the widths of 2.30 and 1.26 eV are determined from the absorption spectral analysis. These band tails shift up to 2.32 and 1.44 eV when the interface is constructed. In addition, an indirect energy band gaps (Eg) which are located at 3.10, 2.13 and 1.90 eV for the MgO, Ga4Se3S and the Ga4Se3S/MgO layers, respectively, are determined. The Eg value of the crystal shifts by a 0.23 eV upon bilayer construction. The reflection properties, the band tails, the energy gaps and related shifts make the Ga4Se3S/MgO interface attractive for fabrication of solar cells, narrow barrier resonant tunneling diodes or quantum dots, and as an optical detector for tunable types of lasers. (C) 2013 Published by Elsevier B.V.