In the current debate on pedagogical content knowledge (PCK), the term is used to refer to the context‐specific knowledge that teachers activate when reflecting on practice. Against the background of this debate, we conducted an empirical study and sought to answer the question of which knowledge experts and novices activated in assessing a videotaped lesson in relation to its effectiveness for learning. Our assumption was that the participants activate their PCK as a blending of content knowledge (CK) and pedagogical knowledge (PK) as suggested by Shulman's amalgam thesis. The participants (9 experts and 9 novices) were shown a lesson on optics, in which the law of refraction (Snell's law) was being studied. In a subsequent interview, the participants were asked to analyze the observed lesson. A qualitative and quantitative evaluation of the interviews showed that experts activated both CK and PK intensively and in this respect they differed significantly from novices. Further analysis of the expert statements also proved that they do not activate their CK and PK in isolation, but instead combine both kinds of knowledge together, in line with Shulman's amalgam thesis. © 2017 Wiley Periodicals, Inc. J Res Sci Teach 9999:XX–XX, 2017