This study describes the use of the double-slit thought experiment as a diagnostic tool for probing physics teachers' understanding. A total of 9 pre-service teachers and 18 in-service teachers with a variety of different experience in modern physics teaching at the upper secondary level responded in a paper-and-pencil test and three of these teachers were interviewed. The results showed that the physics teachers' thought experiments with classical particles, light, and electrons were often partial. Many teachers also suffered a lack of the basic ideas and principles of physics, which probably hindered thought experimenting. In particular, understanding the ontological nature of classical particles, light and electrons seemed to be essential in performing the double-slit experiment in an appropriate way. However, the in-service physics teachers who had teaching experience in modern physics were more prepared for the double-slit thought experiment than the pre-service teachers. The results suggest that both thought experiments and the double-slit experiment should be given more weight in physics teacher education, even if experience in modern physics teaching at upper secondary school seems to some extent to develop teachers' abilities.