In optical mineralogy, the ability of anisotropic minerals to split plane-polarized light into two rays as it passes through them. The rays vibrate at right angles to each other and travel along different path lengths within the mineral, depending on the degree of birefringence encountered. As a result, when the two sets of rays emerge from the mineral one set has travelled farther than the other and is retarded. The retardation shows itself as a range of interference colours when the analyser is inserted, and may be determined by reference to an interference colour chart. See michel–lévy chart.