This paper is focused on primary trainee teachers' conceptualization of the vision process and image formation in a plane mirror. The study involved 55 non-specialist, undergraduate trainee primary teachers on a 4-year programme of initial teacher training. The process incorporated tracking trainees' ideas during university-taught sessions through collating and analysing responses to set tasks that included both the interpretation of annotated diagrams of the vision process and diagrammatic representation of image formation in a plane mirror. A selected sample of trainees was also interviewed. The data indicate that trainees experience significant difficulties in articulating coherent explanations regarding basic ideas about light. This presents particular professional constraint within the current demands of the primary initial teacher training science curriculum. It is argued that a more productive approach would be to focus on the professional issue of pedagogy through raising trainees- awareness of the conceptual difficulties in learning rather than the current curriculum focus that seems to privilege knowing over understanding.