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In this research, the physics tools developed is the determinant of a refractive index of ATMega328 microcontroller based liquid with Ultrasonic Sensor. By utilizing the technology, the experiment becomes easier and simpler so that the experiment will not take a long time. This study aims to: (1) make the Design of Physics Learning Practical Tool determinant of a refractive index of liquid assisted by microcontroller based ultrasonic sensors ATMega328 (2) explain the working principle and experimental result of liquid refractive indexing instrument assisted with ATMega328 microcontroller based ultrasonic sensor. The working principle of the liquid refractive index determiner assisted by the microcontroller based ultrasonic sensor of ATMega328 is to determine the refractive index by measuring the positive lens focal distance (f_1), the combined lens distance between the positive lens and the liquid (f_com) through the reflection of the shadow and radius Positive lens (R). This distance measurement uses ultrasonic sensors as a digital distance measuring device. Determining the refractive index of the liquid (n) by placing the liquid between the flat mirror and the convex lens. The clearest shadow distance captured by the screen is the combined focal distance between the positive lens and the liquid lens (f_com). After the positive lens focal distance (f_1), and the combined lens (f_com), the focusing distance of the liquid lens (f_2) can be determined. The focusing distance of the liquid (f_2) and the fluid lens radius.