In the present article, the context-dependency of student reasoning is studied in a context of optics. We investigated introductory students' explanations about the behavior of light when different light sources, namely a small light bulb and a laser, were used in otherwise identical task assignments. The data was gathered with the aid of pretest and post-test questions and semi-structured interviews. According to the results, the different light sources triggered different sets of students' ideas about light and its behavior. The students' ideas corresponded to the perceptible features of the light sources, and textbook presentations used at the earlier levels of education. The use of the ideas hindered students' abilities to apply the scientific models of light in a coherent manner. Overall the present study suggests that students' difficulties in understanding the behavior of light are partly caused by their ideas about light sources commonly labeled in the task assignments of optics.