Polarization by Scattering

Sunlight is also partially polarized by a second, quite different mechanism: SCATTERING off the tiny air molecules and dust particles that make up Earth's atmosphere. When a beam of sunlight strikes such a molecule or particle, its direction of travel is changed (scattered) -- and this same scattering is exactly why the DAYTIME SKY appears blue rather than black (blue light, having a shorter wavelength, is scattered by air molecules far more strongly than the longer-wavelength red end of the spectrum, so the diffuse blue light reaching our eyes from all directions of the sky, away from the Sun's own direct disc, is this preferentially-scattered blue component). Without any scattering at all, the sky would appear completely dark except when looking directly at the Sun, and stars would remain visible even in broad daylight. …