We discuss a study of student reasoning difficulties and approaches while solving problems about the
brightness of non-identical light bulbs connected in series and parallel. The questions about the
light bulbs can be solved quantitatively even though they were posed as conceptual problems. We
compare the performance of introductory physics students with that of a set of physics PhD students
and find that these problems related to non-identical light bulbs are difficult even for PhD
students. We also conducted individual interviews with six introductory students to obtain an
in-depth understanding of their approaches and rationale for solving the problems in a particular
way. We discuss the conceptual difficulties displayed in the interviews and in the written responses
in which introductory physics students were asked to explain their reasoning. In addition to
confirming some misconceptions which have previously been observed in the context of equal wattage
light bulbs, use of …