The refractive index varies with the wavelength (color) of light, so the positions of the optic axes in biaxial crystals, and the angle 2V between them, will change when the color of the incident light is changed. This phenomenon is usually expressed in the form r > v, indicating that 2V is greater for red than for violet light, or vice versa.[1] For vauxite r > v, 2V is greater for red light than for violet light.