Labels Used to Identify Perpendicular and Parallel Components

When polarized light is incident on a surface, it is often described in terms of perpendicular and parallel components. These are orthogonal to each other and the direction in which the light is propagating (Figure 1).

Labels and symbols applied to the perpendicular and parallel components can make it difficult to determine which is which. The table identifies, for a variety of different sets, which label refers to the perpendicular component and which to the parallel.

The perpendicular and parallel directions are referenced to the plane of incidence, which is illustrated in Figure 1 for a beam reflecting from a surface. Together, the incident ray and the surface normal define the plane of incidence, and the incident and reflected rays are both contained in this plane. The perpendicular direction is normal to the plane of incidence, and the parallel direction is in the plane of incidence.

The electric fields of the perpendicular and parallel components oscillate in planes that are orthogonal to one another. The electric field of the perpendicular component oscillates in a plane perpendicular to the plane of incidence, while the electric field of the parallel component oscillated in the plane of incidence. The polarization of the light beam is the vector sum of the perpendicular and parallel components.

Normally Incident Light
Since a plane of incidence cannot be defined for normally incident light, this approach cannot be used to unambiguously define perpendicular and parallel components of light. There is limited need to make the distinction, since under conditions of normal incidence the reflectivity is the same for all components of light.