Cognitive structure is a structure exhibiting the mutual associations of concepts recorded in the long-term memory. Students set up cognitive bridges between their previous knowledge and new knowledge during the creation of the cognitive structure. Learning a concept does not mean learning what a concept is called or learning only its definition, but it also means being able to set up the associations between the concept and the whole. This study determines high school students' cognitive structures about the concept of light by using a two-stage word association test. Content analysis was used in analysing the data collected. Content analysis is a very effective method yielding accurate results in reaching the concepts, correlations and logical patterns which can describe the data. The data obtained were analysed by using the technique of semantic relations in order to be able to set up logical ties between the number of words and words. 136 high school students (88 female and 48 male) were included in the research. The words having semantic proximity were divided into five categories by analysing the data through word association test. Consequently, the most used categories were as in the following, respectively: Structure of Light (223), Propagation of Light (143) and Properties of Light (142). The findings obtained in this study indicate that high school teachers and university instructors should take special care with some points in the teaching of subjects related with light. Besides, the formation of scientific knowledge about the concepts in individuals' minds will also encourage the emergence of complex organised knowledge about the phenomena.