The current reform efforts in Croatia in the field of physics teaching are aimed at promoting inquired-based teaching of physics, which should replace the current predominantly teaching in secondary schools. The new approach should involve many more student research experiments than is the case now. It is therefore important to explore the effect of their involvement and to assess the possible benefits as well as problems. The research will focus on wave optics, which is taught in the fourth grade of secondary school (ages 18-19). Students will be selected from general secondary schools in the Zagreb area. Existing research points to many students ’conceptual difficulties and suggests that this topic is difficult to master well without experimentation. An additional problem of the lack of appropriate experimental equipment in most secondary schools, as well as the lack of physics teachers with experience in conducting such teaching, is a major obstacle to the introduction of research-oriented teaching in Croatian secondary schools. The aim of this project is therefore to provide the necessary equipment, training, and support for the teachers involved, and then to assess the impact of the teaching intervention on the scientific reasoning and conceptual understanding of secondary school students.