Rupert Murdoch

Physical description

General information

One can observe the effect of a rainbow all the times where there is Eau in suspension in the Air and which a source of light (in general the Sun) shines behind the observer. The most spectacular rainbows take place when half of the sky opposed to the Sun is darkened by the clouds but that the observer is at a place where the sky is clear. Another common place where one can see this effect is near water falls.

Many colors

In occident, one usually considers that it is composed of seven colors: red, orange, yellow, green, blue, indigo and purple. Actually, an arc in sky breaks up only into 6 dominant colors. The Indigo corresponding only to one very narrow band of the visible spectrum, one cannot distinguish it in a rainbow. The usual decomposition 7 colors is due especially to the fact that the number Seven is considered in a very positive way in the Occidental culture. All the people did not find 7 colors to him, nor the six; for Dogons, it has 4 of them; black, red, yellow, green (see further section Mythologies and symbolic system .

Current formation

The rainbow is caused by the Dispersion of the sunlight by roughly spherical drops of rain. The light is initially refracted by penetrating the surface of the drop, undergoes then a reflection partial with the back of this drop and is refracted again while leaving. The total effect is that the entering light is mainly considered towards the back under a Angle of approximately 40-42°, independently of the size of the drop. The precise value of the angle of reflection depends on the Wavelength (the color) of the components of the light. The blue light which is refracted has a greater angle than the red light, but because of the total reflection, the red light appears higher in the sky and forms the external color of the rainbow.

A rainbow thus does not have really a physical existence but is a optical Illusion whose apparent position depends on the position of the observer. All the drops of rain refract and reflect the sunlight in the same way, but only the light of a small portion of the drops of rain reaches the eye of the observer. It is the image formed by the light of these drops of rain that we see in the form of rainbow.

Alternative formations

In rare cases, a rainbow can be seen of night in clear weather and full moon. In this case, it is the the Moon which is used as source of light. In practice, the light of the arc thus produced is weak and can not excite sufficiently the cells of the Rétine responsible for the perception of the color (the cones). The arc appears thus of a gleam grisâtre without apparent color. The colors can however appear on a photograph.

One can also artificially create this effect one day shone upon while turning back on the sun then dispersing water droplets in the air in front of oneself (during a watering for example) the arc is then all the more visible as the bottom is dark.

Position and Size

A rainbow is always located contrary to the sun: sun, the observer and the center of the Circle to which belonged the rainbow are on same the line.

A rainbow always belongs to a of the same circle Diamètre: a circle appearing under an approximate angle of 40-42° around this line sun-observer-center of the arc.

But taking into account the fact that the horizon mask usually most of a rainbow, it is the size of the visible arc which varies: the closer the sun is to the horizon, the more the arc will be large. An observer in high-altitude will see a larger rainbow than an observer with the sea level (especially because he will see most of the sky, and practically not because of the change of alignment with the sun which is tiny).

Of a plane one can see the whole circle of the rainbow with the shade of the plane (giving the direction opposed to the Sun) in his center.

Secondary arcs and supernumerary arcs

Sometimes, a second rainbow, less luminous, can be seen above the primary education arc. It is caused by a double reflection of the sunlight inside the drops of rain and appears under an angle of 50-53° in the direction opposed to the Sun. Because of the additional reflection, the colors of this second arc are reversed compared to the primary education arc, with blue outside and the red inside, and the arc is less luminous. This is why it is more difficult to observe. A third rainbow can be present in the vicinity of the second, and reversed compared to this one (thus identical to the first). It is however definitely less luminous and observable only under exceptional conditions. In practice, it is not very easy to distinguish from the supernumerary arcs associated with the secondary arc (see below). It corresponds to the luminous rays having undergone five reflections in the water drops. Two reversed arcs one compared to the other can also be observed in the opposed direction, with approximately 45 degrees of the Sun (thus in the direction of this one), but this is particularly difficult because of proximity of the Sun. The rare observations of these two arcs intermittently mention piece of arcs visible. These two arcs correspond to the luminous rays having undergone three and four reflections in the water drops. As they are located contrary to the Sun, in fact the same water drops contribute to it. In practice, the configurations favorable to their observation are definitely fewer than those favorable to the observation of the secondary arc, in particular because of their proximity of the Sun.

Another effect less difficult to observe is that of the arcs known as supernumerary, which result in the fact that the first arc appears in fact like a series of arcs of ray, thickness and intensity decreasing coupled the ones with the others. Visually, one observes a copy of the first arc located just inside this one: beside the band violet of the first arc, one observes the green band then the band violet of his copy, thus sometimes that a second copy (see photo opposite). This phenomenon results from interferences undergone by the light at the time of its successive reflections in the water drops (see,). They cannot be explained by only the geometrical Optique, from where their name. Contrary to the other arcs, these supernumerary arcs depend on other factors, like the dispersion of the diameter of the water drops.

The band sinks of Alexandre

Between the first and the second rainbow, a darker band appears. That corresponds to the zone of the water drop ranging between the angle of 42° characterizing the end of the first and the angle of 50° characterizing the beginning of the second. This intermediate band where there is deficit of light was called the " band sinks of Alexandre" , in the honor of Alexandre d' Aphrodisie which described it the first.

History of discovered its formation

The role of the refraction of the light on water droplets was suspecté at the beginning of XIVe century by Persan Al-Fârisî and Dietrich von Freiberg.

The historian anthropologist Michel Pastoureau, specialist in the colors, reminds to us that, in the texts as in the images since antiquity until the Middle Ages, the rainbows have three, four or five colors, but never seven, because the spectrum was not yet known.

Rene Descartes redécouvre the principle of the law of the refraction in 1637. Isaac Newton specifies calculations in second half of the XVIIe century.

Mythologies and symbolic system

The arc in sky, perhaps because of its beauty and the difficulty in explaining it - before the treaty of Galileo on the properties of the light - for a long time seems to have fascinated the Man. It should be noted that all the people do not find 7 colors to him; for Dogons, it has 4 of them; black, red, yellow, green which is the trace left by the mythical celestial ram which fertilizes the sun and urinates the rains.

The rainbow was regarded as a “Pontifex” by many myths and legends founders, and this in the whole world (at the Pygmy S of Africa, in Indonesia, Mélanésie), at some Amerindian S or in the old Japan (where it is named the " floating bridge in the sky ". The arc in sky is presented like a bridge or way. It can be borrowed by gods, Chaman S, Sorcier S or of the legendary heroes, to circulate between the ground and another world or two points far away from the ground.

In the Greek Mythology, this way between sky and ground was created by the messenger ( Iris which one finds trace in the Spanish term: “ arco iris ”.
Dans the Scandinavian Mythology, named Bifröst (or bridge of byfrost; Trembling way), it is the “bridge” which makes it possible to join Ásgard, the kingdom of the gods.
Pour the Irish old, the secret hiding-place of the gold of the Leprechaun Irish was where the end of the rainbow is posed.
La Bible mentions it like signs engagement of God with humanity. “ And God known as: Here the sign of alliance that I put between me and you and all the living beings which are with you, for the generations to come: I put my arc in the cloud and it will become a sign of alliance between me and the ground ”.
En Asia, it evokes also the arc of Çiva, which resembles the rainbow according to the texts, whereas the arc of Indra is the name which one gave to the rainbow with the Kampuchea (Indra produces the lightning and exempts the rain).
Il evokes also the illumination of (Bouddha, which goes down again of the sky by this staircase to the 7 colors, whose slopes are 2 snakes (Nâga).
Les ribbons carried by the Shaman S Bourate S, symbolizing the rise of the spirit of the Shaman towards the sky is called rainbow. Many myths associate also the arc in sky with a mythical snake (or groups snakes).
La Chinese Mythologie presents it like a slit in the sky, sealed by the goddess Nuwa who for that used stones of seven different colors. Five Chinese characters at least indicate the rainbow. All contain the radical “Hoei” , that of the word snake.
Au Tibet, it is not a bridge but the hearts of the sovereigns who join the sky.

In the Symbolic system Occident ale, the rainbow (by what heralding the good weather after the rain?) is often associated with the joy and cheerfulness or with the renewal (an arc in sky appears in the sky at the Insane-hi time of the birth of , and at Chibcha (Colombia), it protects the expectant mothers.
Ses seven colors are present in the Islamic esotericism (image of divine qualities of the universe), and in India and Mésopotamie they represented the seven levels of the skies . Its colors were retained for the banner of the gay The origin of this symbolic system could be that all the colors are joined together there (hot and cold, ying and yang for the Chinese). The number seven is in addition frequently regarded in a positive way and as a crowned figure (cf seven days of the creation of the world, week, the seven notes of music, the seven seas, seven arts, etc).

However certain traditions, of which with Divination, associate it with a danger come from the sky.
Certains people of Central Asia and the Caucasus allotted the capacity to him to aspire the water of the rivers and the lakes, to even carry children or men, ground towards the sky or in the clouds.
Il can also announce the illness or death (ex: among mountain people of south-Vietnam, or in the Pygmies, who regard it as dangerous snake of the sky, or a double welded snake).
Il can announce political difficulties (“ When a state is in danger to perish, the aspect of the sky changes… An arc in sky shows ”; Hoed Nan-tseu).
Pour Négritos Semang the rainbow is a python which shines of all the colors when it goes up to the sky to take a bath, but the water of its bath which it reverses in rain on ground is very dangerous for the human ones. For the Negritos Andaman, it is the Tam-tam (because it is often associated with the thunder?) Spirit Forest , predicts of dead or disease Former Peruvians did not look at the rainbow, and covered their mouth of a hand, because it is also the crown of feather of Illapa (cruel and intractable God of the thunder and the rains). For Incas, the rainbow was a mythical celestial snake. collected by the men in the shape of a small worm. It became gigantic by eat, this why it should have been killed because it imposed that one gives him human hearts to eat. The bright colors of certain birds come owing to the fact that their ancestors soaked themselves in the blood of this giant snake.
Les Indian Pueblos named rainbow the scale giving access their underground temples, evoking this time the bond between the chtonien field and the ground.

Flag with the colors of the rainbow

The flag with the colors of the rainbow is nowadays associated with various concepts:

with the red in top (7 colors): flag of the Empire INCA (1250 - 1548)
with the purple one in top (7 colors): with the movement of the pacifist in Europe (used for example in the demonstrations in Italy against the war in Iraq of 2003)
with the red in top (6 colors): with the gay movement and lesbian, mainly with the the United States.

References

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