This article investigates the optics conceptions of 228 Thai high school students in grades 10-12 when they attempted to explain how an object can be seen both in general and in a completely dark room. To elicit students' conceptions precisely, their confidence in giving reasons for the final two diagnostic questions is also considered. The study indicates that these high school students still have beliefs and misconceptions about how an object can be seen and how the human eyes work. Only 6.57% (15 students) of all participants could use scientific principles and strong self-confidence to explain how an object can be seen and how human eyes work, answering all questions correctly and with appropriate reasons. The identified misconceptions are discussed and compared to the literature. The teaching implications for developing a conceptual understanding of how human eyes work under low light intensity levels are suggested and discussed.