|Laser used as part of the Adaptive Optics System of the ESO telescopes|
(C) ESO - Babak Tafreshi - All rights reserved
|Rayleigh sky geometry|
- When the sun is located at the zenith, the band of maximal polarization wraps around the horizon. Light from the sky is polarized horizontally along the horizon. (note: the horizon is at 90 degrees related to the zenit)
- When the sun is near the horizon, the maximum polarization is, again at 90 degrees from the horizon, or in the Zenit.
- Note that because the polarization pattern is dependent on the sun, it changes not only throughout the day but throughout the year. (note: the apparent position of the sun changes during the day)
- Many animals use the polarization patterns of the sky at twilight and throughout the day as a navigation tool. Because it is determined purely by the position of the sun, it is easily used as a compass for animal orientation. By orienting themselves with respect to the polarization patterns, animals can locate the sun and thus determine the cardinal directions. (Bees and birds would have a huge problem on contrary)
- As the sun sets due West, the maximum degree of polarization can be seen in the North-Zenith-South plane. Along the horizon, at an altitude of 0° it is highest in the North and South, and lowest in the East and West. Then as altitude increases approaching the zenith (or the plane of maximum polarization) the polarization remains high in the North and South and increases until it is again maximum at 90° in the East and West, where it is then at the zenith and within the plane of polarization.
So, we have some interesting situations:
- Sun at Zenit: Maximum polarization (MaxPol) on a 360 degrees circle at the horizon.
- Sun at Horizon, let's say to the east. MaxPol is at a semicircle from north to south, passing through the Zenit. MinPol points are at east and west near the horizon.
- For other directions just follow the same rule, but don't forget that the angles and vectors are on a sphere.
There are two types of polarizer filters, linear and circular. Linear polarizers are far more efficient to polarize the light from the sky than the circular one.
This can be a good or a bad thing depending on how do you want to enhance or attenuate the polarization. Linear polarizers tend to make the effect stronger, then sometimes the banding sky will be more visible or less visible depending on the polarizer type used at the moment
Circular polarizers are needed when your camera system has a mirror. This is because the mirror will modify the polarization and some light meter and auto focus systems sensors are placed behind a semi-silvered mirror. This difference in polarization may impact on the readings.
In case you're interested in a more detailed explanation, please check this page at Lindsey Optics website. Bob Atkins also has a very cool explanation on his website.
There are other aspects that can change the sky color, like pollutants and dust. It's a well-known phenomenon that the sun gets redder at the sunrise and sunset. This is due to the combination of two things, scattering and absorption.
Larger particles in suspension in front of the sun will block the smaller wavelengths (the blue-purple part of the solar spectrum).
This phenomenon is also the cause of why do we perceive the sun color as yellowish, caused by the short wavelengths scattering by the extremely small particles and absorption by the larger ones. The Sun looks white from space.
Scattering and absorption are extremely common processes also in deep space. Looking at nebulas, you will see both processes and also a third one called emission.
|Lagoon Nebula in Visible light (left) and infrared (right)|
Hubble Space Telescope - (C) NASA
The image at the right was taken using an infrared sensor and it looks completely different. Infrared light wavelength is much longer and the very small dust particles are much less efficient on blocking it. This is why we can see through the dust cloud and see this myriad of stars.
Some of the red/pink light is caused by the ultraviolet light from the star ionizing the hydrogen present in the nebula.
And, to complete the frame, the light is also polarized!