Polarization axis

The polarisation axis is the direction in the plane perpendicular to k along which the electric field oscillates for a linearly polarised wave. It is a vector — specifically, a unit vector ê — defining one of the two independent degrees of freedom for a transverse wave. For a polariser (Polaroid sheet, wire grid, crystal) the polarisation axis (often called the transmission axis) is the direction along which the incident E-component is passed through; the perpendicular component is absorbed or reflected.

When linearly polarised light encounters a polariser, the transmitted intensity follows Malus's law: I = I₀ cos²θ, where θ is the angle between the incident polarisation axis and the polariser's transmission axis. When unpolarised light passes through a polariser, half the intensity is lost (averaged over all input polarisations), and the output is linearly polarised along the transmission axis. Rotating a polariser and observing the transmitted intensity of an incoming beam — the classical ellipsometry test — directly reveals the polarisation state of the beam. The polarisation-axis concept generalises in elliptical polarisation to the orientation of the semi-major axis of the polarisation ellipse.