The research presented in this dissertation looks at how students in general chemistry reason about atomic emission spectroscopy. Situated within the context of a transformed general chemistry curriculum called "Chemistry, Life, the Universe, and Everything" (CLUE), the goal of this study was to (1) characterize the various ways in which students explain atomic emission spectra, and to (2) refine the treatment of spectroscopy concepts within CLUE to improve students' understanding of light-matter interactions. Using a design-based research methodology, a series of evidence-based curriculum changes were implemented within CLUE from Fall 2013 to Fall 2016. The specific changes that were made during each phase of this study were directly informed by observations of student understanding from the prior year. To assess the effect that these changes to curriculum and assessment had on students' reasoning, a robust coding scheme was developed to characterize the extent to which students link and integrate their knowledge of spectroscopy concepts. In F13, students were interviewed to gain insight into how they understand atomic spectroscopy. Findings showed that students had difficulty explaining the mechanistic process for how spectral lines are created. To address this issue, the curriculum materials (i.e. homework, recitation activities, summative assessment tasks, and instruction) were refined in F14 to provide a more explicit emphasis on the mechanistic process by which an atomic emission spectrum is created. These changes led to an improvement in the percentage of students who reasoned about electronic transitions; however, there was no improvement in the number of students who reasoned about energy quantization. Based on this observation, the formative and summative assessments were refined in F15 to emphasize the quantized nature of energy. However, these changes did not lead to any observable differences in students' reasoning. To see if a more explicit question prompt would better elicit a more detailed explanation of atomic emission spectra, only the summative assessment task was changed in F16. Findings showed that the summative assessment task used in F16 elicited more sophisticated reasoning. Based on an analysis of four separate cohorts of general chemistry students who were enrolled in CLUE during different phases of curriculum refinement, it appears that general chemistry students' explanations of atomic emission spectra ranged from simple descriptions of properties of light to highly complex responses in which students applied their understanding of the mechanistic process for how an atomic emission spectrum is created to explain how different colored spectral lines are produced or why each element has its own unique emission spectrum. The effect that the curriculum and assessment changes had on students' reasoning is presented within this study. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]