Many sources of light emit unpolarized light. Light from the Sun is unpolarized, as is light from old style athode ray tubes and light emitting diodes. On the other hand light from lasers is completely polarized because the production of light is stimulated by the presence of photons of light with energy corresponding to the the difference between certain energy levels in atoms. Photons in a laser beam all have the same phase and polarization as a result of the process of stimulated emission.
Unpolarized light can be partially polarized by reflection. Light photons have magnetic and electric fields oscillating at right angles. These fields moving together constitute a light ray. Interaction with a atoms or molecules can rotate the fields. Partial polarization of light occurs when one orientation of the electromagnetic fields is preferred over others. This can occur during reflection when light of certain polarizations are removed. When light reflects off a surface, light waves that have the electric field oriented in the plane of reflection – p polarized light - defined by the incident and reflected rays is more likely to be refracted. Light waves with electric fields oriented perpendicular to the plane of reflection (s-polarized light) are more likely to be reflected. The net effect is that reflected light is partially polarized.
It is possible to use reflection off a surface to isolate s polarized light. If the orientation of the incident ray of light and the surface are adjusted carefully, one can find an angle at which all of the reflected light is s polarized and none of it is p-polarized. This angle at which no p polarized light reflects is called Brewster’s angle and can be found for any pair of media forming a surface where
and 
are the indices of refraction for the two mediums: