The study determines the conceptual understanding levels of primary school students on the concept of light according to the Rasch Model with a Four-tier Light Conceptual Understanding Test (LCUT). The participants were 355 (164 girls and 191 boys) primary school students studying at a public school in Izmir city center. In the study, the Rasch Model, which is included in the Latent Trait Theory, was used. Also, the data regarding the answers given and the level of confidence in the responses were associated with the Rasch analysis of LCUT. The results of Rasch analysis showed that LCUT was in full harmony in the context of infit, outfit, and point measurement correlation statistics, and is a valid and reliable measurement tool for conceptual understanding. Moreover, these results explained that the students' average conceptual understanding ability regarding the Light unit was above the average item difficulty.