The rainbow – The perfect wonder of light, water and tons of physics!

Boats sail on the rivers, And ships sail on the seas; But clouds that sail across the sky Are prettier than these.

There are bridges on the rivers, As pretty as you please; But the bow that bridges heaven, And overtops the trees, And builds a road from earth to sky, Is prettier far than these. - Christina Rossetti

_{Source: pixabay.com}

Hello everyone, It's good to be back. I had a bit of serious school work to get outta the way for the past week. Well, I've knocked that off my bucket list, and today, I'll be considering a wonder in our sky – The rainbow.–The beautiful streak of colours we eagerly wait to observe after rainfalls.

Don't we all just love colours? Without colours, sight would be so awful that you and i wouldn't be able to tell the difference between the colours black and blue, or differentiate yellow from brown. But thanks to the electromagnetic spectrum of visible light, we could see and use them to connote signs and meanings. there's red for caution signs and greens for trees and environments. There's green and white for my country's flag and there's red, white and blue for the stars and stripes! –So there, colours are everything!–.

The rainbow has always posed a captivating and beautiful art of nature that has held man spell-bound from times old. Such is its beauty that Christina Rossetti says, “There are many pretty sights in this world, some of which are manmade, but nothing compares to the beauty found in nature. The beauty of clouds and rainbows cannot be replicated by human hands”.

Wikipedia defines the rainbow as a meteorological phenomenon that is caused by the reflection, refraction and dispersion of light in water droplets, resulting in a spectrum of light appearing in the sky. It takes the form of a multicolored circular arc.

Where exactly do rainbows come from?

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Oh wait, I know; All of the lights!

_{Source: Pixabay}

Yup, that's correct; Our sun is at it again!

The sun's ray is a very bright white light made up of the wavelengths of the visible spectrum. Light has a dual personality, and what that means is that it occurs as an electromagnetic wave, as well as particles we termed as "Photons". The visible light spectrum is made up of "colours". But the truth is that light has no colour. Rather, they are wavelenghts of different magnitudes. Colours are just our brain's way of interpreting these wavelengths. Here's a photo of the electromagnetic spectrum, with visible light expanded

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Light from the sun is all white!, However, our sky scatters shorter to bigger wavelength colours from the visible spectrum when light travels through it. During the day, it scatters violet and blue colours, leaving a yellowish hue around us, – Hence, the reason why sky is blue and sunlight is viewed afar off as yellow. During mornings and evenings, the sun appears reddish because then, The rays need to travel longer distance in the atmosphere which causes scattering of yellow light too, and gives a red coloration to the sun when viewed afar off.

Thought corner:

Observing a piece of white paper outside during the day, we find that it looks white, because it recombines the sunlight that comes through our atmosphere with the scattered blue from that sunlight. If the sun were actually yellow, then white paper would look yellow and not white.

Obviously, at a speed of about 300,000km per second, It's no news that light travels way faster than sound in air and vacuum, and that it travels a bit slower in liquids, and doesn't travel through opaque materials at all. When light is passed from one medium of propagation to another – Say, from air to liquid, – it slows down, and then as a result of that, something interesting happens; "LIGHT BENDS"!, and changes its direction ever so slightly in the new medium! Science has termed this bending as Refraction. Physics discusses the refraction of light through glass prisms, and as evidenced in the picture below, this refraction allows light to be spread out into its spectrum of colours!

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How the rainbow forms, and why the colors are so arranged as ROYGBIV.

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Unlike the constant speed of light in vacuum, the speed of light in water depends on some factors such as the colour of the light. Hence, the constituent colours of white light as a whole, doesn't refract at the same rate. Red light travels faster in water than purple (which is at the other end of the visible light spectrum). Hence, red travels and bends faster in water. This causes the spectrum of white light to spread out, exposing the various seven colours, each arranged according to their speeds. This "spreading out" of light in a medium is called "Dispersion".

Well, now we've known how a change in medium causes dispersion of light, but, where's the water in air that causes this refraction and dispersion? The answer lies on the insides of the water droplets in our sky! These droplets act as prisms and when sunlight passes through them, the wavelengths in white light are refracted by the drops to reveal the colors of the rainbow. Water droplets make crappy mirrors in that, they do not obey the 'angle of incidence equals the angle of reflection' rule, and the degree to which they deviate depends on the frequency of light.

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When entering a droplet of water, light is refracted, and then reflected inside on the back of the droplet at an angle of 42° as maximum deflection for red light and 40.89° for purple light, and refracted again when leaving it. The other colours present are also reflected at various angles in between the maximum boundaries for Red and Purple.

With the numerous rays of white light refracting at the surface of the air droplet at different points and angles, it normally would be a mess, and would just cancel out to give off white again. However, each colour has a maximum intensity at their maximum angles of reflection, thus, they shine brighter at such angles, overshadowing the mess of colours caused by other less bright colours to make it white.

With multiple light rays reflected inside the bubble, and the rest refracted outwards, the insides of arc of a rainbow appears brighter

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This explains a lot about why the air underneath the rainbow appears brighter than the air outside of its arc. And also, it explains why the colours are so arranged as red being at the top of the arc, and purple underneath. Light of primary rainbow arc is 96% polarized tangential to the arc.

_{Image source: pexel.com}

As i have previously noted, it is:

• Refraction upon entry, then;

• Reflection at the enclosure of the insides of the droplets, and finally;

• Refraction again on exit!

Rainbows can be caused by many forms of airborne water. These include not only rain but also mist, spray, and airborne dew. The rainbow effect is also commonly seen near waterfalls or fountains.
For rainbow colors seen by the human eye, the most commonly cited and remembered sequence is Newton’s sevenfold Red, Orange, Yellow, Green, Blue, Indigo and Violet (ROYGBIV).

Sometimes, we see another arc of colours just outside of the primary arc of the rainbow, with inverted colours ( that is, red underneath and purple on top). Why's that?

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Well, this phenomenon is called the double rainbow, and is caused by the light being reflected twice on the inside of the droplet before leaving it. Light of secondary rainbow arc is 90% polarized.

A rainbow has no beginning and no end, so it's a misconception to want to "find the end of a rainbow." Good luck with that. Again, I would state that it isn't an object either. One cannot touch the rainbow. It's simply an interaction of light and moisture in our sky. –A Wonder, as it were–

Before now, I have often reasoned, is there even a reason why the rainbow is called a "rain-bow?" I mean, it doesn't appear as a zap of colours like the lightening bolt, neither does it look anything like a straight line. Why is a rainbow an arc?

Actually, it's called a "bow" because we could only see a part of the full arc of a rainbow. A rainbow is actually a complete circle, but the horizon cuts off the rest of it. If the sun were a bit lower than normal on moist days, we'd most likely see a greater part of the circle.

With the sun behind you, and raindrops in front, what you see is the coloured light from any of the raindrops which happen to be in the right place to refract the colours back to you.

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Thought corner

One way we could verify this is by climbing on a stepladder and spraying fine drops of water with a hose. By conducting this simple experiment, we could easily see the entire circle, and if we are lucky, we could see the secondary rainbow too.

Here's a photo of a full rainbow made by a user on quora.com on his driveway with 120 micron glass beads.

Source: quora.com

In the photo, It is observed to be completely round and always centered around the shadow of the head, - the anti-solar point-.

It is also intriguing to find that rainbows aren't absolute to everyone, but rather unique. This would mean that everyone sees "their own rainbow", regardless of whether we're seeing the colours being reflected off the same water droplet or not. What matters is that it is always an arc, and I find this simple fact of individual uniqueness in rainbows to be rather magical.

From the few paragraphs above, it can be said that two things are always needed to create a rainbow;

We need the Sun low in sky (during sunsets or mornings) and water droplets in the air.

Ancient myths about the rainbow.

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From Greek mythology to ancient Romans, Rainbows have been a part of various myths in diverse cultures and peoples. there are countless stories and explanations of what the rainbow actually is.

In Greek mythology, the rainbow was believed to be a path made by a messenger Iris between earth and heaven, whereas the Chinese believed that the rainbow was a slit in the sky sealed by goddess Nuwa using stones of five different colors.

The Norse people thought of the rainbow as a bridge between Asgard (home of the gods) and Earth. They believed that the rainbow could be attained by the good and virtuous people.

The Irish leprechaun’s secret hiding place for his pot of gold was usually believed to be at the end of the rainbow.

Some other Europeans believed that the gender of anyone who could pass beneath the arc of the rainbow would be transformed. – Implying that it could change man into woman and woman into man!

In summary,

• Rainbows are images but certainly not objects. They are simply seen as a result of a poor reflection of the Sun's rays in the moist atmosphere. Poor, but beautiful.

• A rainbow is not located at a specific distance from the observer but comes from an optical illusion caused by any water droplets viewed from a certain angle relative to a light source. Thus, a rainbow is not an object and cannot be physically approached.

• Rainbows appear in seven colors because water droplets break sunlight into the seven colors of the spectrum.

• Nobody can ever reach the end of the rainbow, because as you move, the rainbow that your eyes see moves as well, because the raindrops are at different spots in the atmosphere. The rainbow then will always move away at the same rate that you are moving.

So the next time you're considering or counting the wonders in the sky, don't you ever forget the rainbow. It truly is magical!

Thank you for reading,

Yours,

@pangoli

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