In the simplest form, uniaxial birefringence, there is only one special direction in the material. This axis is known as the optical axis of the material. Light with linear polarization perpendicular to this axis will experience an "ordinary" refractive index "n"o while light polarized in parallel will experience an "extraordinary" refractive index "n"e. The birefringence of the material is the difference between these indices of refraction, Δ"n" = "n"e − "n"o. Light propagating in the direction of the optical axis will not be affected by the birefringence since the refractive index will be "n"o independent of polarization. For other propagation directions the light will split into two linearly polarized beams. For light traveling perpendicularly to the optical axis the beams will have the same direction. This can be used to change the polarization direction of linearly polarized light or to convert between linear, circular, and elliptical polarizations with waveplates.