Two models for a physical situation : the case of optics. Students' difficulties, teachers' viewpoints and guidelines for a “ didactical structure ”
Philippe Colin
The first part of the study focuses on difficulties that students, at university level, encounter in analysing physical situations implying two models - geometrical and wave optics. These difficulties seem to be strongly connected with misreading the drawings linked to a lack of awareness about their status. In order to overcome these students' difficulties, the content analysis is based on "backward selection" of paths of light : a situation is analysed starting from the considered arrival point of the light. This main feature, not found in the textbooks, leads to breaking the rigid link between device and model to be used and to an in-depth analysis of the very common notions of optics (ray, source, object, image). 
To provide some guidelines for the design of innovative teaching, an investigation of teachers' viewpoints is needed to maximize the teacher-acceptability of the proposals. The thematic analysis of teachers' interviews reveals on the one hand, the obstacles that some aspects of the proposals will probably encounter and, on the other, the anchoring points on which an innovative teaching strategy can rely. 
On these results, the proposals are graded on a scale of teacher-acceptability. From the bottom to the top of the pile, different layers are added to backward selection. Guidelines, including not only new lightening of the content of physics but also clues for teaching strategies (gradation of difficulties, choice and use of situations) are provided to teachers so that they can build an adapted innovative sequence.