Before revision: Because there is no p-polarized reflection from the pure liquid when both are angled towards it at the Brewster angle, light is only reflected when some other phenomenon such as a surface film affects the liquid surface.
After revision: (oscillating along the plane formed from the axis of light travel and the vertical normal of the surface)Because there is no p-polarized reflection from the pure liquid when both are angled towards it at the Brewster angle, light is only reflected when some other phenomenon such as a surface film affects the liquid surface.