2026ApJ...998..127L

Growing evidence indicates that some fast radio bursts (FRBs) reside in dense, magnetoionic environments where extrinsic propagation effects can substantially reshape the observed signal. Within a 1D Gaussian plasma-lens framework, we show that small, monotonic variations in the incidence angle of the FRB wavefront naturally generate both downward and upward sub-burst frequency drifts. We further demonstrate that distinct lensed paths that probe different rotation measures (RMs) can produce orthogonal polarization-angle (PA) jumps at gigahertz frequencies. In this picture, a ∼90° PA transition requires only a modest RM contrast of order a few ×10 rad m^–2 between the multiple images. The chromatic activity of FRB 20180916B-earlier and narrower activity windows at higher frequencies-can be explained as preferential magnification near the outer caustic. Finally, the intrinsic resolution of a plasma lens provides an upper limit on the transverse emission size: lenses located close to the source yield magnetospheric-scale constraints and offer a practical means of discriminating between inner- and outer-magnetospheric emission scenarios. These results suggest that plasma lensing could account for multiple complex observational features of FRBs and may play a nonnegligible role in modulating their observable properties.