Students' understanding to learn science on the concept of light refraction is experiencing serious problems because of the unobservable of light particles. The research object to be resolved in this study is to improve the concept of light refraction using a photodiode sensor. The problem to be solved in this research is to develop a Simple Kit (SK) Refraction of Light (RoL) using a photodiode sensor for students' understanding of concepts. The research method for making SK RoL using photodiode sensors using ADDIE, which consists of the first stage, analysis by analyzing the needs of SK RoL products using photodiode sensors to be developed. Second, the SK RoL design uses a photodiode sensor. Third, Development is developing SK RoL Using Photodiode Sensors. Implementation of the Fourth RoL Decree Using Photodiode Sensors to see student understanding of 74 students from tertiary institutions in one of the provinces in Indonesia. Fifth, Evaluation by revising the product at each stage of SK RoL development. SK RoL product research results include Green Laser, Photodiode sensor, Angle Arc, Display Module and Displayed Data, Arduino Uno R3 Atmega 328, Medium Container and Programs Using Arduino Software. The characteristics of the SK ROL results show that the SK RoL based on the Photodiode Sensor developed can be used as a physics learning medium because it can visualize the unobservable concept of light refraction and increase students' understanding of the concept of light refraction because of the nature of the particles which are difficult to understand by using a green laser which is passed through a photodiode sensor medium and Arduino can be visualized in a real way. SK RoL can work optimally in a room where the light is not too bright or requires a room that is a bit dark so it is possible to observe the laser beam. SK RoL, as a physics learning medium, can be used to increase students' understanding of the concept of light refraction. The implications of developing SK RoL can be used as a learning medium to attract students' interest in learning, support the learning process, and help make it easier for students to understand abstract physics concepts

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