Non-conservative scattering force field (black arrows) generated by a collimated Gaussian beam along the propagation z-direction (arrow at the bottom). The grey arrows represent the gradient force field. The total work done by the scattering force depends on the path of integration and is different in the cases of paths l1 and l2. Instead, the work done by the gradient force is independent of the path; i.e., the gradient force is conservative.
Fig. 3.9 — Scattering force