An unpolarised light beam strikes a glass surface at Brewster's angle Then

When an unpolarised light beam strikes a dielectric surface (such as glass) at a special angle called Brewster’s angle, the reflected and refracted (transmitted) rays have distinctive polarisation properties.

Brewster’s angle (θ_B) is defined by the condition that the reflected ray and the refracted ray are perpendicular to each other. This condition leads to the well‑known relation

where $n\_1$ is the refractive index of the medium the light is coming from (air, ≈1) and $n\_2$ is the refractive index of the second medium (glass, ≈1.5). Solving for θ_B gives the angle at which the reflected light becomes linearly polarised.

At θ_B the electric‑field vector of the reflected wave lies entirely in the plane perpendicular to the plane of incidence (the “s‑polarised” direction). The component of the electric field that oscillates in the plane of incidence (the “p‑polarised” direction) is completely suppressed in the reflected beam. Hence the reflected light is **completely polarised**.

The refracted (transmitted) ray, however, still contains both s‑ and p‑polarised components. The p‑component continues to obey the usual Fresnel transmission coefficient, while the s‑component is reduced but not eliminated. Therefore the transmitted beam is **partially polarised** – it is not fully polarised because a residual s‑component remains.

Summarising the result:

At Brewster’s angle the reflected light is completely polarised, whereas the refracted light is only partially polarised.