s-polarization

s-polarisation (from German senkrecht, "perpendicular") refers to an electromagnetic wave striking an interface with its electric field vector perpendicular to the plane of incidence — the plane containing the incoming k-vector and the surface normal. In waveguide and antenna literature the same state is called TE (transverse electric) polarisation because E is entirely transverse to the plane of incidence.

For an s-polarised wave hitting a dielectric interface, the Fresnel reflection coefficient r_s = (n₁cosθ₁ − n₂cosθ₂)/(n₁cosθ₁ + n₂cosθ₂) is always negative (assuming n₂ > n₁) and never crosses zero — s-polarised light always partially reflects at an interface, with the reflection fraction rising monotonically from the normal-incidence value to 100% at grazing. This is why glare off a water surface or glass window is preferentially s-polarised: the p-polarised component is mostly transmitted (especially near Brewster's angle) while the s-component is reflected. Polarising sunglasses and camera filters have their transmission axis vertical precisely to block the horizontally polarised s-reflections from horizontal water and road surfaces.