Unpolarised light strikes a glass surface at Brewster's angle ($\mu = \tan i_p$). The reflected light is:

At Brewster's angle the reflected and refracted rays are perpendicular to each other. Light polarised in the plane of incidence (the electric vector parallel to that plane) is not reflected at all because the radiating dipoles in the medium are oriented exactly along the direction the reflected ray would have to travel. So the only component that survives in the reflected ray is the one perpendicular to the plane of incidence. Result: reflected light at Brewster's angle is completely plane-polarised, with vibration perpendicular to the plane of incidence. The refracted ray contains both components and is therefore only partially polarised.