This article reports on the results of a study involving an innovative assessment program initiated to investigate student predictions and revised explanations regarding a variety of optical phenomena. The assessments were administered via videotape to two classes of high school physics students from different high schools. Two high school teachers with similar educational and teaching backgrounds administered the tapes to the two groups of students. The school environments and the ability levels of the two student groups were similar. The students of the teacher with greater experience with this new form of assessment provided considerably more substantive explanations to the phenomena presented on the videos. The results of the case study suggest that the introduction of new forms of assessment in science education, although desirable, will require significant reordering of the goals and strategies of science teaching.