The incidence and prevalence of alternative conceptions in science have been reported by a number of researchers. This article reports findings of a study involving the development and implementation of two intervention programmes to address alternative conceptions related to interference and diffraction of waves among 133 first year university physics students. The two interventions were (1) a learner-centred approach based on the principles of outcomes-based education (OBE), and (2) a teacher-centred approach in the form of a traditional lecture, characterised by frontal dissemination of information while students passively listened and took notes. A total of nine alternative conceptions had earlier been identified in the pilot part of the study. In determining the effectiveness of the interventions, a pre-test/post-test comparison group design was followed. The results revealed statistically significant gains between the pre- and post-test scores for both the OBE and traditional lecture groups (p < 0.05). Qualitative data showed that most of the pre-existing alternative conceptions appeared to have been alleviated, although not completely overcome, by both interventions. There was no statistically significant difference between the results of the OBE and traditional lecture interventions. The implications of these findings are discussed, and recommendations are made for effective teaching and learning, as well as for further research.