As light passes from one substance or medium to another it is refracted, that is, the transmission direction is changed. The velocity of light is a funciton of the medium through which is passes, being slower in a more dense mediium. Since the frequency of vibration of light remains constatn, the wavelenght must change as light enters a different medium to accommodate the change in velocity. The change in wavelength upon entering a different medium gives rise to refraction, or the bending of light. By definition, birefringence is a numeric value, not a visible property.
Interference color is the visual manifestation of birefringence using polychromatic light.
Although the interference color may change, the birefringence of a mineral remains constant. Interference colors are produced only in anisotropic minerals under cross-polarized light. When the grain is not at extinciton, the mineral will divide the plane-polarized light into rays vibrating at right angles to each other. These two rays, whe resolved along the vibration direction of the analyzer will interfere. The interference is due to the fact that the two rays travel trough the mineral with different velocities and wavelengths.