Let There Be Rainbow Light

I have a rainbow machine!

Let there be rainbow light

Ooohhh…

Let there be rainbow light

Did you know that a rainbow…

Let there be rainbow light

Wait!

Let there be rainbow light

Let there be rainbow light

You can tell the story now 🙂

Let there be rainbow light

Hmmm…. Did you know that a rainbow has always been associated with creation, divinity, good luck, duality, peace, energy and even gates for extraterrestrials worlds? The rainbow has been a favorite component of mythology throughout history.

In Norse mythology, a burning rainbow bridge called the Bifrost connects Midgard (earth) with Asgard, home of the gods. And of Thor! 🙂 Bifrost can only be used by gods, special bears and those who are killed in battle.

Bifrost
Bifrost

In the ancient beliefs of Japan, rainbows were the bridges that human ancestors took to descend to the planet.

In Navajo tradition, the rainbow is the path of the holy spirits, and is frequently depicted in sacred sand-paintings.

The Maori tell a tale of Hina, the moon, who caused a rainbow to span the heavens even down to the earth, for her mortal husband to return to earth to end his days, since death may not enter her celestial home.

In Greek mythology, Iris is the personification of the rainbow and a messenger linking the gods to humanity. Like many Greek gods, Iris was continually being redefined and the rainbow eventually became solely a mode of transportation for Iris, who was as elusive and unpredictable as the rainbow itself. The iris of the eye is named after her to reflect the many colours of the eye.

Anuenue, the rainbow maiden, appears in Hawaiian legends as the messenger for her brothers, the gods Tane and Kanaloa.

The rainbow is depicted as an archer’s bow in Hindu mythology. Indra, the god of thunder and war, uses the rainbow to shoot arrows of lightning.

The Epic of Gilgamesh, who was an ancient Sumerian king (ca.3000 BCE), is our first detailed written evidence of human civilization. In a Victorian translation of a Gilgamesh variant, Leonidas Le Cenci Hamilton’s Epic of Ishtar and Izdubar, King Izdubar sees “a mass of colors like the rainbow’s hues” that are “linked to divine sanction for war.” Later in the epic, Izdubar sees the “glistening colors of the rainbow rise” in the fountain of life next to Elam’s Tree of Immortality.

In Australian Aboriginal mythology, the rainbow snake is the Creator (Kurreah, Andrenjinyi, Yingarna, Ngalyod and others) in the Dreaming, which is the infinite period of time that began with the world’s creation and that has no end. People, animals, and Eternal Beings like the Rainbow Serpent are all part of the Dreaming, and everyday life is affected by the Dreaming’s immortals, in almost every Australian Aborigine tribe. In these tribes, actual rainbows are gigantic, often malevolent, serpents who inhabit the sky or ground. This snake has different names in different tribes, and has both different and similar traits from tribe to tribe.

Rainbow Serpent
Rainbow Serpent

Long ago in the Dreamtime when the earth lay sleeping and nothing moved or grew, lived the Rainbow Serpent. Then one day the Rainbow Serpent awoke and come out from beneath the earth. Refreshed from her long slumber she travelled far and wide leaving winding tracks from her huge body and then returning to the place she had first appeared.

On her return she called to the frogs “come out!” The frogs came out slowly as their bellies were full with water which they had stored during their long sleep. The Rainbow Serpent tickled their stomachs and when the frogs laughed, the water spilled out all over the earth to fill the tracks of the Rainbow Serpent. This is how the lakes and the rivers were first formed.

With water, grass and trees began to grow which woke all the animals who then followed the Rainbow Serpent across the land. They were happy on earth and each lived and gathered food with their own tribe. Some animals lived in rocks, some on the vast plains, and others in trees and in the sky.

The Rainbow Serpent made laws that they were all to obey but some began to make trouble and argue. The Rainbow Serpent said “Those who keep my laws will be rewarded; I will give them human form. Those who break my laws will be punished and turned to stone & will never to walk the earth again”.

Those who broke the law became stone and were turned into mountains and hills and those who were obedient were turned into human form and were each given their own totem of the animal, bird or reptile from when they began. The tribes knew themselves by their totems – kangaroo, emu, carpet snake, and many, many more. So no one would starve, the Rainbow Serpent ruled that no man should eat of his totem, but only of other totems. This way there was food for everyone.

The tribes lived together on the land given to them by the Rainbow Serpent or Mother of Life and knew the land would always be theirs, and no one should ever take it from them.

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On a serious photonic note, a rainbow is an optical phenomenon caused by interaction of light, normally the sun, with water droplets, normally after the rain. In general, when light passes from a medium to another one, it splits in two parts: the reflected and the refracted. Let us imagine, now, that the first medium is the sky, and the second one is a water droplet. When the sunlight encounters a raindrop, part of it is reflected and another part is being refracted at the surface. When this light hits the back of the drop, once again reflection and refraction happen. And if we follow just the reflected one, it will be again reflected and refracted again when trying to escape the raindrop.

Interaction of light with a rain droplet
Interaction of light with a rain droplet

Without entering in too much detail on the physics of rainbows, the overall effect is that part of the incoming light is reflected back over the range of 0° to 42°, with the most intense light at 42°. This angle is independent of the size of the drop, but does depend on its refractive index.

Pink Floyd, Dark Side of the Moon
Pink Floyd, Dark Side of the Moon

Like in the Pink Floyd cover album The Dark Side of The Moon, showing the light dispersion in a prism, in a raindrop the amount by which light is refracted depends upon its wavelength, and hence its color: blue light is refracted at a greater angle than red light, but due to the reflection of light rays from the back of the droplet, the blue light emerges from the droplet at a smaller angle to the original incident white light ray than the red light. That’s why you will always see blue on the inside of the arc of a rainbow, and red on the outside. If the raindrops are big enough, and light reflects twice inside them, you will see a secondary rainbow at an angle of 50–53°. In this case colors are inverted compared to the primary one, with blue on the outside and red on the inside.

First and Second order Rainbow in Norway, 2009. Credits: Antigone Marino
First and Second order Rainbow in Norway, 2009, by Antigone Marino

Rainbows are to some extent in the eye of the beholder. All raindrops refract and reflect the sunlight in the same way, but only the light from some raindrops reaches the observer’s eye. This light is what constitutes the rainbow for that observer. The whole system composed by the sun’s rays, the observer’s head, and the spherical water drops has an axial symmetry around the axis through the observer’s head and parallel to the sun’s rays. This explains the circular arc-shape of the rainbow: whatever is the effect of any water’s drop on the observer, rotating around the axis must leave it unchanged. Therefore, the bow appears to be centered on the shadow of the observer’s head, or more exactly at the antisolar point (which is below the horizon during the daytime, unless the observer is sufficiently far above the earth’s surface), and forms a circle at an angle of 40–42° to the line between the observer’s head and its shadow. As a result, if the sun is higher than 42°, then the rainbow is below the horizon and usually cannot be seen as there are not usually sufficient raindrops between the horizon and the ground, to contribute.

Two “circular rainbows" in Nyalam, Tibet,  by Antigone Marino
Two “circular rainbows” in Nyalam, Tibet, by Antigone Marino

The “circular rainbow” is actually a halo. It is created by reflections and refractions on hexagonal ice-crystals, instead of water drops. What happens close to the border between Nepal and Tibet is that the warm and wet air coming from the Nepalese subtropical valley goes up, reaching at the Nyalam altitude the cold Himalayan climate. In these conditions it is easy to have ice-crystals formation in the troposphere.

Rainbows and halos both rise from diffraction phenomena. The former between sunlight and water droplets, the latter between sunlight and ice-crystals. Halos can have different shapes (pillar, spots and arc), while rainbows appear as arches. In circular halos, the size is constant, determined by the angle of refraction through six-sided ice crystals. The primary halo is always 22°. If color is visible, it will be red on the inside and blue on the outside. The secondary halo, twice as wide, has reversed colors.

We saved you a cupcake!

Let there be rainbow light

2015 has been declared the International Year of Light (IYL).

International Year of Light 2015 Logo
International Year of Light 2015 Logo

While a number of activities have already taken place, the official launch will be at the UNESCO headquarters in Paris on 19 January 2015, with the unveiling of 1001 Inventions and the World of Ibn Al-Haytham. This is a global campaign that includes a series of interactive exhibits, workshops and live shows about Ibn Al-Haytham’s achievements in optics, mathematics and astronomy, and his importance in laying the foundations of the present day scientific experimental method. The campaign will be produced by the 1001 Inventions organisation.

The major scientific anniversaries that will be celebrated during IYL 2015 are:

1015: Works on optics by Ibn Al-Haytham
1815: The notion of light as a wave proposed by Fresnel
1865: The electromagnetic theory of light propagation proposed by Maxwell
1915: Einstein’s theory of the photoelectric effect in 1905 and of the embedding of light in cosmology through general relativity
1965: Discovery of the cosmic microwave background by Penzias and Wilson, and Charles Kao’s achievements concerning the transmission of light in fibers for optical communication

An amazing number of events have been organised worldwide. You will find all the details here:

http://www.light2015.org/Home/Event-Programme.html

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