<![CDATA[Purpose of the study: This study aimed to design, refine, and evaluate Photonics Explorer Kit (PEK)-supported inquiry activities for enhancing preservice physics teachers’ conceptual understanding of optics, particularly in reflection, refraction, and diffraction. Methodology: The study employed Educational Design Research (EDR) involving two iterative cycles with 89 level 200 preservice physics teachers at the University of Education, Winneba, Ghana. Cycle 1 involved 51 participants, while Cycle 2 involved 38 participants. Data were collected using a validated two-tier diagnostic test and analysed using descriptive statistics, paired-samples t-tests, effect sizes, and comparison of learners’ correct conceptions, partial conceptions, misconceptions, and no-explanation responses. Main Findings: The findings showed significant improvement in participants’ conceptual understanding in both cycles. In Cycle 1, mean scores increased from 7.75 to 11.14, t(50) = 25.78, p < .001, d = 1.95. In Cycle 2, mean scores increased from 8.10 to 12.46, t(37) = 29.84, p < .001, d = 2.34. Cycle 2 also recorded higher correct conceptions and fewer misconceptions across the assessed optics concepts. Novelty/Originality of this study: The study presents an Educational Design Research-based approach for refining PEK-supported 7E inquiry activities in optics. It shows how iterative refinement of instructional support, diagrams, inquiry questions, and multimedia can strengthen preservice physics teachers’ conceptual understanding in optics, especially in resource-constrained physics teacher education contexts. ]]>
