Schematic diagram of refraction by a Gaussian plasma lens (after Clegg et al., 1998). The dashed lines show lines of constant phase before and after a plane wave passes through a Gaussian lens. The lens is shown by a thick solid line. The representation of this lens is purely schematic, as it is assumed to have a negligible and uniform width along the line of sight. The thin solid lines represent ray paths which are normal to the phase front after the wave has passed through the Gaussian lens. Focussing occurs at the focusing points. The interference of the refracted waves causes variations in the signal intensity on relatively small spatial scales in the grey-shaded region, which is bounded by the inner and outer caustics.