The study aims to identify secondary school ninth-grade students’ previous knowledge and misconceptions about plane mirrors. To this end, the data were collected by using “the Conceptual Understanding Test on the Unit about Mirrors” and “Semi-structured Interview Forms”. The conceptual understanding test consists of a total of four questions about “image formation”, “image characteristics” and “visual field”, the titles under the unit on plane mirrors. The test was administered to a total of 310 students from two different high schools. In evaluating the conceptual understanding test, the researchers first wrote down the correct responses to the questions and categorized the students’ responses as “codable”, “uncodable” and “no response”. Codable responses were further grouped under the categories “scientifically acceptable” and “scientifically unacceptable”; and the data were then analyzed. As a result of the examinations on the data obtained from the conceptual understanding test and the interviews held with a total of 16 students, students were determined to have misconceptions about “the relationship between observer’s position and image”, “the position of image on a plane mirror”, “the relationship between the object and image”, “image characteristics”, and “the factors upon which visual field depends”.