Light is used ubiquitously across science and engineering to explore, characterise, understand matter, and catalyse processes. Relative to its utility in science and engineering, very little research has been conducted on how students develop an understanding of light-matter interactions, especially at the quantum level, which is necessary to understand many modern applications. To begin addressing this gap, we present results from a cross-sectional qualitative investigation of postsecondary chemistry students' conceptions of light and light-matter interactions in the context of the photoelectric effect. A knowledge analysis of the interviews resulted in a model of students' understanding that consisted of three distinct levels of knowledge. These levels were distinct from each other based on the features of students' cognitive structure and treatment of wave-particle duality. The lowest level reasoning constructed fragmented explanations and relied on intuition; and described light behaviour as only a wave or a particle. In contrast, the highest level reasoning coordinated observations and inferences to construct mechanistic explanations of the photoelectric effect; and described light behaviour as both wave and particle. These results provide a starting point for characterising how students conceptually transition from the classical, macroscopic view to the quantum view.