Understanding students’ conceptions and difficulties has a very important role for improving quality of physics teaching. Wave optics concepts are very abstract and complex, and they are not a part of student’s everyday life. Motivated with the aforementioned facts, we conducted a qualitative research on nine high school students from one general type gymnasium and one gymnasium specializing in science and mathematics from Zagreb. We tested students’ knowledge after they had completed regular classes on wave optics. The research was conducted in the form of semistructured interviews that included topics of polarization, double slit and optical grating interference, and single slit diffraction of light. The results were analyzed, classified by students’ difficulties and compared with the previous research of the group at the University of Washington. The identified students’ main difficulties related to the concepts of interference, diffraction and polarization suggest that inquiry-based teaching, in which experiments play a central role, would be important for the teaching of wave optics. In addition to the student difficulties found in previous studies, several new difficulties were also found. Student difficulties were classified, explained and accompanied by students’ quotes and drawings from the interviews.