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Looking at the blue sky using my polarizing sunglasses I notice an effect (details below), that seems to match well to this answer, where it is said:

The greatest degree of polarisation occurs when the scattering angle is through 90 degrees (since one of the polarisation states of the incoming solar light is effectively "unseen")

My question is about the reason for the effect.

Experience:

At early morning, if I look at the West using my polarized sunglasses, and tilt my head up, the darkest blue colour is about the zenith. And if I start looking at the North or South and repeat the head movement, the zenith is brighter. It suggest that the light coming from the zenith is the most strongly polarized.

When I do the same test at noon, but now looking at the South, (the sun about 30 degrees North), the darkest region is somewhere between the zenith and the horizon.

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  • $\begingroup$ Yes. I did not notice the link at the final of the answer. $\endgroup$
    – Claudio Saspinski
    Commented Aug 8, 2020 at 11:45
  • $\begingroup$ BTW, this effect is quite handy when looking at the Moon at First Quarter or Last quarter phase, since it's 90 degrees from the Sun, so looking at it through a polarizer really improves the contrast, especially in the daytime. $\endgroup$
    – PM 2Ring
    Commented Aug 8, 2020 at 18:15

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The root cause is that you see no radiation from an oscillating electric dipole when looking along the axis of oscillation.

Unpolarized light can be considered an equal mixture of two perpendicular linear polarisation states (with random phases). Such light causes electric dipoles in the atmosphere (i.e. molecules) to oscillate in those two perpendicular directions and emit the Rayleigh-scattered light. If this scattered light is observed along a direction parallel to one of these perpendicular oscillation axes, then no light will be seen from that oscillating component, leaving just pure linearly polarised light from the other oscillation component.

Clearly, this happens when the light is scattered through 90 degrees.

The situation is more complicated in Earth's atmosphere, because the scattering takes place at a range of heights, there is the possibility of multiple scattering, and there are also particles larger than the wavelength of light (aerosols and dust) that have differing polarising properties.

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  • $\begingroup$ Yes the situation is indeed more complicated, but in daytime clear sky Rayleigh first scattering dominates and yields a good polarization contrast. In these conditions aerosols' contribution is really noticeable only in the vicinity the solar disk, in about 5° area, which is far from the 90° where we can see the maximum polarization. $\endgroup$
    – Ruslan
    Commented Aug 8, 2020 at 19:30

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