Fig. 1. Polarization arising from light scattering within the earth’s atmosphere. Unpolarized sunlight (upper left panel) remains unpolarized if it reaches the observer directly (scattering angle 0°, right panel), but is linearly polarized if it is scattered by atmospheric O₂ and N₂ molecules. Within a theoretical (Rayleigh) atmosphere, the degree of polarization reaches 100%, if the scattering angle is 90° (lower left panel). Other scattering angles yield smaller degrees of polarization (lower right panel). The light is then said to be partially linearly polarized. In the real atmosphere, the degree of polarization – even in full blue skies – is almost always less than 70% (see Horvath and Wehner, 1999). Background landscape: Naukluft gravel plain desert, north of Gobabeb, Namibia.