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The purpose of this research is to develop the instrument of diagnostic test on optical geometry material, especially in the mirror and lens topics, as well as to unveil the misconceptions of students at one of the public schools in Subang Regency. A purposive sampling technique sampling with sample selection utilizing cluster random sampling technique was conducted on a total of 96 respondents. The instrument utilized by modifying the Four Tier Diagnostic Test which was developed by Kaltakci- Gurel (2017) is developing diagnostic test on geometrical optics (DT-GO). The results of the instrument analysis on the validity test, reliability test, difficulty level test, and discriminatory power, state that the instrument was very well utilized and developed. Based on data analysis and discussion, it can be concluded that overall misconceptions had occurred of overall misconceptions that occur in students are 42.04 %, thus they meet the requirements for misconceptions to a moderate level. [ABSTRACT FROM AUTHOR]