Light consists not only of specific colours spectrum, but lightwaves also have a preferred direction of oscillation. Light is a quickly moving (electromagnetic) wave. The colour of light is determined by the oscillation frequency of the wave. The plane in which the wave oscillates is the polarization direction of the wave. This is illustrated below.
The top panel shows a wave moving in the direction of the long arrow. This wave is polarized in the vertical direction. The bottom panel shows a wave that is polarized in the horizontal direction.
Direct sunlight is unpolarized; the light has no polarization direction. As the sunlight passes through our atmosphere, it is scattered by air molecules, water, and tiny atmospheric particles that we call aerosols, causing the light of the blue sky to have a well-defined polarization pattern with specific polarization orientations that depend on the location of the sun in the sky and the part of the sky that is observed. This is why the blue sky is (partially) polarized. The same scattering process, known as Rayleigh scattering, also causes the sky to be blue.
The polarization state of skylight is not directly visible to the human eye, but can be perceived as slight brightness and color differences in clear blue skies. This effect is most strongly observed at points on the sky that are at a 90° angle to the sun (at this scattering angle the linear degree of polarization is at a maximum). This effect of (partial) polarization can be used to darken the sky in photographs and thereby increase the contrast, as is shown below. Photographers use polarizing filters to do this. Another application of polarizing filters are polarized sunglasses. The polarizing filters in these glasses remove specular reflections – e.g. the glimmering on a water surfaces – and thereby increase the contrast.
The polarization is characterised at each wavelength by its degree of polarization and its polarisation direction or orientation. The degree of linear polarization as seen from the ground is perpendicular to the line from the Sun to the location of that particle. The degree of linear polarization depends strongly on the type, size and composition of the scattering particle. In other words, the particles leave a unique imprint on the spectrum and polarization of the light that they scatter. By measuring the scattered sunlight spectrum and amount and direction of polarization, a method called spectropolarimetry, we can determine the type, size and composition of particles.
iSPEX uses spectropolarimetry for the measurement of atmospheric aerosols.