When unpolarised light is incident at a particular angle such that the reflected and refracted rays are perpendicular to each other, this angle of incidence is called Brewster's angle ($i_B$). According to Brewster's law, $\mu = \tan i_B \implies i = \tan^{-1}(\mu)$. At this angle, the reflected light is completely plane-polarised, and its electric field vector is perpendicular to the plane of incidence.