This research was intended to investigate whether Turkish students who had graduated from high school used the wave theory of light properly in explaining optical phenomena. The survey method was used in this research. The data, which were collected from 175 first year university students in Turkey, were analysed quantitatively and qualitatively. Eight open-ended questions were used as data collection tools. These questions were related to what the students understand about the scientific theory of light and how they apply the wave theory of light to the phenomena of refraction, double slit interference and single slit diffraction. The data showed that although the students were aware of the scientific theory of light, they developed alternative conceptions of the application of the wave theory. This situation caused students to have incorrect ideas about optical phenomena. These ideas and their results are discussed and some recommended solutions are presented in the last part of this article. (Contains 7 tables and 5 figures.)