Since this varies with the color of the light, a standard color must be chosen when refractive indices are specified. The usual standard is the yellow light from a sodium source, that has wavelength 589.3 nanometers. For an anisotropic substance the refractive index (for light of a given color) varies with direction,[2] and for vauxite the range is from 1.551 for light travelling parallel to the a axis to 1.562 for light travelling parallel to the c axis.