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Previous work has highlighted the difficulties students have when explaining wave behavior. We present an investigation of chemistry students’ understanding of the double-slit experiment, where students were asked to explain a series of PhET simulations illustrating a single continuous light source, single-slit diffraction, and double-slit interference. We observed a variation in student reasoning and students were categorized into groups based on their ability to explain and generate a mechanism for the double-slit experiment. Some students struggled to explain the features of waves which impacted their reasoning about interference and caused them to rely on intuition to generate explanations. Other students were able to productively incorporate their previous knowledge about wave behavior, with their observations from the simulations, to build a robust mechanism for wave interference. However, students generally exhibited a limited understanding of interference, and specifically attending to the key features of waves during instruction can promote more sophisticated reasoning about this phenomenon.