You might have seen a rainbow, that arch of colours visible in the sky, but do you know what it is?
 

What is a rainbow?

 
A rainbow appears when sunlight and water meet. They are caused by the refraction and dispersion of the sun's light by rain or other water droplets in the atmosphere. Rainbows can be seen not just when it's raining but also on misty or foggy days, in the spray of a waterfall, and even in dew, basically they can appear whenever there are water droplets in the air and there is sunlight shining from behind at the right angle.  It's called light refraction, and on a sunny day, with a hosepipe, you can even create your own rainbow in the garden.
 

A full rainbow is actually a complete circle, but from the ground we see only part of it, which is why it's an arc.  In fact, given the right conditions, you could see an entire circular rainbow, but you'd need to be very high up in the air.

The colours of the rainbow are generally said to be red, orange, yellow, green, blue, indigo, and violet (the spectrum).  We often say, "all the colours of the rainbow". These colours are created by 2 things: 

  • Sunlight is made up of the whole range of colours "wavelengths" that the eye can detect. When combined, the range of colours look white to the naked eye, what we call sunlight.  This property of sunlight was first demonstrated by Sir Isaac Newton in 1666.
  • When the light of all these different colours is refracted in different ways, as it passes from one medium (air, for example) into another (water or glass, for example) the colours separate, ranging from violet on the bottom to red on the top.

If you're very lucky, you might see a double rainbow.  In certain conditions you can see another, fainter secondary rainbow above the primary rainbow. The primary rainbow is caused from one reflection inside the water droplet. The secondary rainbow is caused by a second reflection inside the droplet, and this “re-reflected” light exits the drop at a different angle. This is why the secondary rainbow appears above the primary rainbow. The secondary rainbow will have the order of the colours reversed, too, with red on the bottom and violet on the top.

There are many legends from different ancient cultures, all created to explain them, because let's face it, rainbows are among the most beautiful of nature’s displays.

One of the earliest literary mentions of a rainbow is in the bible: Genesis 9, as part of the flood story of Noah, where it was a sign of God's covenant to never destroy all life on earth with a global flood again. 

In the UK they tend to be associated with good fortune, but it's unlucky to look at a rainbow through glass.

In Ireland leprechauns hide their pots of gold at the end of the rainbow. 

There was a belief among ancient Polynesians that a rainbow was a ladder that their heroes climbed to reach heaven.

One old wive's tale says that a house that is over arched by a rainbow will soon experience a disaster, or if you walk through the end of a rainbow, your family will experience a disaster within a year.

In ancient Greece people thought that Iris, wife of the god Zephyrus, caused rainbows. Iris was a messenger between mortals and the gods. She ran back and forth, dressed in shimmering multicolored robes. The word "iridescence" comes from Iris's robes.

The Karens, a group of people in Burma, once considered rainbows to be dangerous demonic spirits that devoured the souls of humans and caused sudden or violent deaths. They thought that such activity made the rainbow thirsty enough to appear in the sky and dip down to Earth to drink water.

Rainbows have been used as a symbol of hope or social change for centuries: The rainbow featured as a symbol of the Cooperative movement in the German Peasants' War in the 16th century, and even in these modern times the rainbow still has the power to move us: Rainbow flags are used as a symbol of peace in Italy, and they have been used as a symbol of gay pride and LGBT social movements since the 1970s. In 1994, Archbishop Desmond Tutu and President Nelson Mandela described the newly democratic post-apartheid South Africa as the rainbow nation.

The fact is a rainbow isn’t really a “thing” and it doesn’t exist in a particular “place,” I'm sorry to tell you, you'll never reach that pot of gold, as you walk towards the end of a rainbow, it will appear to "move" farther away, because a rainbow is simply an optical phenomenon that appears when sunlight and atmospheric conditions are just right, and the viewer’s position is just right to see it, but no one ever sees the same rainbow, which is a kind of magic all its own.

Sources:-

Visually

Wikipedia

Nasa

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