In this study, whether or not constructivist teaching of double-slit interference of light has a positive effect on the secondary school students' conceptual change is examined. An achievement test, a conceptual understanding test and semi-structured interviews were used as data collection tools in this mixed methods research. Experimental group was taught with constructivist approach while control group was taught with traditional method in which teacher was the only authority in defining the course of teaching. It has been identified that experimental group students show high level conceptual understandings after teaching. Superiorities of experimental group, which involve the construction of cause and effect relationships, the transfer and configuration of knowledge, have been identified with the interviews conducted. It was concluded that teaching in the experiment group was more successful than teaching in the control group in terms of the construction of the meaning of interference, path difference, fringe width concepts and understanding the formation of bright and dark fringes. (Contains 2 tables and 6 figures.)