  Wheeler's delayed choice experiment, a well known manifestation of the
complementarity principle, has proved somewhat difficult to physically
interpret. We show that, restated in quantum field theoretic language, the
experiment submits to a simple explanation: that wave- or particle-nature is
imposed not at the slit plane but at the detector system. The intepretational
difficulty conventionally encountered is due to the assumption of enforcement
of complementarity at the former.
