Within an open-gel reactor we study the interaction of spiral waves in a light-sensitive Belousov-Zhabotinskii (BZ) medium by varying the applied light intensity which controls the local excitation threshold. A small, initially symmetric pair of counter-rotating spiral waves after several rotation periods undergoes a symmetry-breaking instability. For a spiral wave interacting virtually with its mirror image on the opposite side of a plane boundary impermeable to diffusion, we measure the drift along the boundary as a function of the applied light intensity. The experimental data are compared with the results of numerical simulations obtained from a modified Oregonator model.