Demonstration interviews with 27 high school students (18-19 years old) were conducted in Zagreb, Croatia, using several standard experiments on interference and diffraction of light. Students were asked for their predictions, observations, and explanations of the experiments. In this process, many student difficulties were identified, both regarding their understanding of interference and diffraction of light, but also regarding their skills of systematic observation and description of experimental patterns. The observed difficulties were analyzed in the resource-based model, suggesting the activation of some p prims, as well as other cognitive resources in the process of students' attempts to predict, describe, and explain the interference and diffraction patterns.