An investigation conducted after standard lecture instruction in a sophomore-level modern physics course revealed that many students were unable to interpret the photoelectric experiment in terms of the photon model for light. Findings from this research were used to guide the development of an interactive computer-based tutorial to address the conceptual and reasoning difficulties that were identified. The primary instructional strategy used in the tutorial is the drawing and interpretation of graphs of current versus voltage for the circuit in the experiment. The program has been used both as an aid to instruction and as a probe to obtain additional information about the nature, prevalence, and persistence of specific difficulties. Analysis of student performance on examination problems on the photoelectric experiment indicates that those who have worked through the tutorial make fewer errors and give better explanations than those who have not had this experience. This result suggests that the intellectual engagement required by the program helps students improve their understanding of the photoelectric effect.