The present study aims to identify first year Turkish Science undergraduates' understandings and misconceptions of the concept of light and its propagation. For this purpose, an instrument composed of four open-ended questions was developed by the researchers. The diagnostic test was piloted with twenty students and modifications were made prior to the final administration of the test. The content validity of the test questions was assessed by two physics researchers and one lecturer. These questions were administered to a hundred first year undergraduates (aged 16-18) enrolled in the Science Teacher Training Department in Ataturk University in Turkey. In addition, the semi-structured interviews of 30-40 min were conducted with the four students whose responses involved common misunderstanding. All interview records were transcribed and analysed. The findings showed that undergraduates' understanding of light is poor and also they have important and prevalent misconceptions. Identified misconceptions, some of which have been reported in the literature, are discussed qualitatively. The misunderstandings identified were compared with those in the literature. The results have some implications for teaching light, suggesting that a substantial revision of teaching strategies is needed.