Undergraduate laboratories are often seen as key components of a university science education. In some disciplines, the hours spent by undergraduate students in the lab are mandated by external accreditation requirements; in others, the development of practical skills is seen as a crucial step in students' development. Despite the level of importance often claimed for labs, in Australia first year labs in particular are typically staffed by PhD students with limited teaching experience or training, employed on a casual basis. The ways in which these casual teaching staff conceptualise learning and teaching will inevitably have significant impact on how they view the undergraduate lab environment, and what opportunities for learning they are aware of and facilitate.
We report on a study investigating the ideas that physics demonstrators have about teaching, learning and the purpose of undergraduate laboratories at three Australian universities. Analysis of interviews with twenty demonstrators revealed two potentially critical threshold concepts with respect to their professional development and approaches to teaching, one connected with perceptions of lab goals and the other with perceptions of student capacity. Demonstrators who clung to a conception of labs in which students were striving to achieve a preconceived correct result were found to undermine the increasingly enquiry-based approach of many lab designs. Demonstrators who believed that students have an inherent and unchangeable capacity to be experimental physicists tended not to value their professional development, and varied their teaching strategies mostly in line with their assessment of a student's pre-existing ability. In contrast, demonstrators who believed students could learn were more likely to value professional development and changed their teaching style as they learnt more about how students learn.