Chromatic effects near a white-light vortex

[12) Leach T.and Padgett M.J., 2003, New ]. Phys., 5 154 increased to make the largest of them fully saturated whilst maintaining their ratios. Figures 2 and 3 show our experimental results. Figure 2 contrasts the case where the optical vortices for each spectral component in the white-light source are overlapping and not overlapping. When the vortices are not overlapping, the chromascope image reveals the chromatic patterns at the center ofthe beam. Figure 3 compares the experimental results to the theoretical predictions made by Berry. In both cases, the transition across the image from red to blue is clear. Note that neither image contains any green. Although the detailed form of these patterns is complicated, the underlying principles, which give rise to these chromatic effects, can easily be understood. For each spectral component, there is a corresponding intensity zero. The dispersion of the vortices produces a line along which specific speetral components are not present. At one extreme, there is no blue component, at the other, there is no red component, and in the center, there is no green. This gives rise to a dominance of the complimentary colours, cyan, magenta (purple) and yellow. The absence of any green region over the entire beam cross-section is explained since it would require the red and blue vortices to be coincident, which they are not.