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This quadruple set of anticoronal rings (i.e. glory) is found around the shadow of a commercial
aircraft. Sunlight is being diffracted through water droplets at the topof a winter Altocumulus layer.
The glory occurs at a point directly opposite the sun, where the (airborne) observer's shadow falls.
(photo by R.F. Reinking)
This is perhaps the most remarkable phenomenon in atmospheric optics. It consists of a coloured aureole that is visible around the shadow of the observer's head. As he has never seen the aureole appear around other people's heads, the unwitting observer may therefore suppose that the aureole is an indication of his being superior to other people! Unfortunately, the fact that others may just as easily observe such an aureole around their own heads, more or less obviates this explanation.
The glory only appears when the shadow falls upon small water drops, such as those in clouds or fog. So, the glory is not a halo. One must be above the fog or clouds in order to see it, which is a rather exceptional situation. Therefore, observations from times past are rather rare. Nowadays the opportunity to see a complete glory is greater, because many people fly by plane above the clouds. However, parts of the glory can also be observed in terrestrial circumstances if one only knows where to look and can achieve the proper situation.
The size of the glory (some degrees in diameter) depends only on the size of the drops in which it is formed and not upon the distance of the observer from the cloud. The smaller the drops, the larger the glory. On the other hand, the size of one's shadow does, of course, depend on the distance from the cloud: so when one recedes from the cloud, the shadow grows smaller but the glory keeps the same dimension. The centre of the glory is exactly where the shadow of one's eyes might be and thus precisely opposite to the sun. In former days pilots used to avail themselves of this property for navigation: instead of 'shooting' the sun, they sometimes 'shot a glory'. Its depth below the horizon corresponds exactly to the desired solar elevation. When flying with the sun at the back, this kind of measurement is more convenient. When one walks through the aeroplane to its front part, it becomes evident how exactly the glory is standing opposite to the sun: the glory is moving with you along the shadow of the aeroplane. Of course everyone on board sees his own glory.
On closer inspection the glory appears like this: a bright white area directly around the shadow, with a red edge, surrounded by some coloured rings with blue inside and red outside. The innermost red edge is brightest. Although becoming progressively fainter, the colours may repeat themselves up to five times. Nearer the Earth's surface the glory can be observed upon low fogs, most successfully when the background is dark (e.g. over an asphalt road). Parts of the glory are also visible in the vapour above a cup of tea, in the 'cloud' breathed out on a cold day, or in the vapour of a bathroom shower. In the last case it is best observed when a thin beam of sunlight is shining into the bathroom. The phenomenon can often be visible from mountains or high towers rising above a layer of fog. The glory is usually seen together with a fog-bow.
The polarization of the glory is very striking. The coloured rings are Radial ly Polarized, hence contrary to the polarization of the fog-bow. This contrast can be clearly observed in the steam of a bathroom shower. However, close to its centre, in the white area, the glory is Tangential ly. polarized; the central point itself has no polarization. This curious contrast is best seen when one can observe a bright glory in its entirety. With a vertically directed polarizing filter the upper and lower sides of the coloured rings become brighter and they look thicker. On the other hand. in the white area close to the centre, again on the upper and lower sides, dark bluish wedge-like triangular spots appear which point to the centre of the glory. Obviously, when the filter is rotated, this structure co-rotates. The triangular spots resulting from the tangential polarization are often more conspicuous than the radial polarization of the rings around them. In the steam of a shower, it is easy to ascertain that the degree of polarization of the glory depends on its size: when the steam has just formed (so the droplets are small) the glory is large and its rings are considerably more polarized than when the glory has become smaller. In principle, it is even possible for unpolarized glories to occur sometimes. They may, moreover, differ from the usual glories by having dark, instead of bright, centres. It is not known if this type of glory often occurs in Nature.
The explanation of the glory is not simple: only since the 1970s has this phenomenon been fully understood. It has emerged that the light consists mainly of two contributions. The first is a light-path with one internal reflection in the drop, in which surface waves are responsible for the fact that the light rays can be scattered backwards. These surface waves cause a radial polarization of the rings of the glory. The second contribution is a ten-fold internal reflection of the light rays in the drops which is related to the formation of the tent rainbow. Such light has a tangential polarization. In small droplets the first contribution dominates; in larger ones the second contribution also becomes important. This explains the decrease of the radial polarization ( the rings of the glory with increasing drop diameter. Interference of light causes a reversed direction of polarization, for both contributions, near the centre of the glory. The net result is a phenomenon with a very curious polarization. In this respect, the glory does not have an analogue 1 atmospheric optics.
Normally speaking, contributions like this (surface waves, light of the tenth rainbow) are very weak and hard to see in Nature. Since, however. the light near our shadow is amplified significantly, it does contribute to the glory. It is remarkable that in this way the tenth rainbow can actually h seen in Nature - albeit irrecognizably changed into the glory an manifesting itself merely as a decrease in the glory's polarization wit increasing drop size.
source: Polarized light in Nature