An understanding of Light-Matter Interactions (LMI) is critical for using and appreciating experimental methods in chemistry as well as the energy crisis facing the world. Because chemists use spectroscopy and light-matter interaction so ubiquitously and spectroscopy appears throughout the entire undergraduate chemistry curriculum, we are in a unique position to support the understanding and faculties learners need to consider pressing problems and emerging technologies (e.g., solar panels, semiconductors, light-harvesting devices, energy storage, etc.). Currently, instruction in chemistry about spectroscopy and light-matter interactions is fragmented and limited by the absence of any systematic investigations into students' understanding. We use a learning progression approach to model students' understanding. Using this approach, we iterate through qualitative interviews about topics within LMI to build a picture of students' cognition, which we organize into a developmental model, and then designing and evaluating assessments to observe and diagnose students' understanding. We do this across a variety of topics until we can build the most comprehensive picture, with evidence from both qualitative investigations and assessment data, to inform curriculum design.