  Boundary conditions (BCs) are considered as an important topic that advanced
physics under- graduates are expected to understand and apply. We report
findings from an investigation of student difficulties using boundary
conditions (BCs) in electrodynamics. Our data sources include student responses
to traditional exam questions, conceptual survey questions, and think-aloud
interviews. The analysis was guided by an analytical framework that
characterizes how students activate, con- struct, execute, and reflect on
boundary conditions. Common student difficulties include: activating boundary
conditions in appropriate contexts; constructing a complex expression for the
E&M waves; mathematically simplifying complex exponentials and checking if the
reflection and transmission co- efficient are physical. We also present
potential pedagogical implications based on our observations.
