NOTE – For a linearly polarized wave, the ellipse degenerates into a line segment and then the plane of polarization is not uniquely defined. In an isotropic medium, the plane of polarization is now conventionally taken to be normal to the direction of propagation. This is a change from previous practice when the plane of polarization was conventionally taken to be the plane containing the direction of propagation and a field vector, generally the electric field vector in radio waves or the magnetic flux density vector in optics.