Interference, diffraction, and polarisation of light are the basic phenomena of wave optics. According to the findings of previous studies on wave optics, these phenomena are the source of many students' conceptual difficulties. The goal of the present study was to qualitatively investigate and compare Croatian and Austrian secondary school students' understanding of the main concepts and phenomena of wave optics as a first step that will serve as a preparation for a large-scale study on secondary school students understanding of wave optics. Here we present the preliminary results of six semi-structured demonstration interviews conducted in Croatia and Austria with secondary school students (age 18-19), after regular school instruction on wave optics.