The main goal of this study was to evaluate guided inquiry (GI) pedagogical strategy for conceptual understanding of eighth-grade students in elementary optics. Simultaneously, it explored and quantified students’ conceptions of elementary optics before and after instructions. The design employed in this study was quasi-experimental research design, which had experimental and control groups (40 students in the experimental and 39 students in control group). The study took place in Atse Serste Dingle primary school in Bahir Dar town, Ethiopia. GI pedagogical strategy was employed for the experimental group students; whereas, conventional instruction (CI) was applied to control group students, for almost two months. The data collected through open-ended conceptual test and simple drawings and analyzed with the facet- scheme hierarchical organization, abundance and gain of it to determine and quantify students’ conceptions. Before instruction, students in both group showed misconceptions which described as nonscientific facets of knowledge. Those misconceptions are almost identical with other students’ misconceptions, which was identified by researchers from western countries. This study revealed students’ conceptions from a developing country: Ethiopia. After CI, students showed little alteration on each facet and scheme of knowledge. However, students who taught with GI pedagogical strategy showed a considerable shift and alteration of each facet and scheme of knowledge than conventional instruction. They magnificently overcame the given nonscientific facets and schemes of knowledge and built scientific concepts. Therefore, GI pedagogical instruction resulted in better students’ conceptual understanding of elementary optics than CI, which is aligned with the results of other studies in physics. However, most of the previous studies measured the conceptual change as a whole with students’ scores of concept test. Determining students’ conceptual change through the shift and alteration of each facet is valuable to identify persistent ideas that need special concern and further treatment. Besides, this study recommended for amendment of the eighth-grade students’ text book on the concept of seeing phenomena and light rays. It also recommended to teachers to apply GI pedagogical strategy in teaching optics. Nevertheless, more studies in different schools, school locations, and on various types of students should be investigated to further generalize the study.