VIBGYOR : Newton’s Rainbow and Indigo

VIBGYOR : Newton’s Rainbow and Indigo

Reading Time: 11 minutes

VIBGYOR is a popular mnemonic for the seven rainbow colours. 450 years ago, Newton split white light into its coloured components and labelled seven of them. Curiously, most people see only six. The Nerd Druid Investigates!

VIBGYOR

One of the very first English words I had learnt was VIBGYOR. Of course, it wasn’t really a word, but it was associated deeply with that thing that all childhood craves, colour.

Violet. Indigo. Blue. Green. Yellow. Orange. Red.

Red is the colour of passion, of love, of anger. Orange tastes sweet and sour, and reminds one of winter, and the Dutch; it is also the colour of greed. Yellow is the brightest of all colours, but is also associated with cowardice, and fear. Green is Life itself, of its great willpower to “…always find a way…“, as Ian Malcolm loves repeating. Blue, the colour of the skies and the seas, is calm, and instills hope. Indigo has a colourful history as a natural dye, and a confused one as regards its place in all this. And when a nor’wester, an April thunderstorm gathers clouds so deep that they look violet, you know the evening’s going to get more interesting.

Image of the Green Lantern emotion spectrum, representing VIBGYOR. Don't be alarmed if the emotions and colours don't quite match what you believe. I, as do most, associate Love with red. This is simply the Green Lantern emotion spectrum, an invention of DC comics. The white logos in the centre of each flag are the symbols of each of the coloured Lantern corps. The most well known are, of course, the Green Lantern Corps, sitting pretty right at the centre, analogues of the Jedi from Star Wars. Image Credit : Unsure, but probably DC comics.
The Green Lantern emotion spectrum, representing VIBGYOR. Don’t be alarmed if the emotions and colours don’t quite match what you believe. I, as do most, associate Love with red. This is simply the Green Lantern emotion spectrum, an invention of DC comics. The white logos in the centre of each flag are the symbols of each of the coloured Lantern corps. The most well known are, of course, the Green Lantern Corps, sitting pretty right at the centre, analogues of the Jedi from Star Wars. Image Credit : Unsure, but probably DC comics.

ROYGBIV

Richard Of York Gave Battle In Vain

Up until a few days ago, I used to think that VIBGYOR was the most common colour mnemonic in the English-speaking world. Turns out this is only true for India. For the US and the UK, the mnemonic of choice is ROYGBIV.

Now, that is of course simply VIBGYOR reversed; the Brits and the Yanks seem to prefer starting off with red. While ROYGBIV doesn’t quite pronounce as sweetly as VIBGYOR (vibjeeyohr), it does make up a (sort of) a name, Roy G. Biv. This is how pre-schoolers in the US learn their colours. The British are far more dextrous; they also have thicker history books. Thus,

Richard Of York Gave Battle In Vain

Richard of York was, of course, Richard III, the last king of England to die in battle. In 1485, he was defeated and killed at the Battle of Bosworth Field, an event that ended the War of the Roses1. The victor, Henry of the House of Tudor, ascended the throne as Henry VII.

Portrait of King Richard III. Artist unknown. Photograph Credit : National Portrait Gallery, London, UK.
Portrait of King Richard III. Artist unknown. Photograph Credit : National Portrait Gallery, London, UK.

Aristotle

Summer in Kolkata

Summers in India are hot. Summers in the Eastern metropolis of Kolkata are hot and very humid. Sweltering and suffocating are two English adjectives that attempt capture a Kolkata summer. They fall well short of the mark.

Curiously, the Kolkata summer this year, 2018, has been uncharacteristically…pleasant. Yes, it has been very hot and yes, it has been very humid. But, interspersed within these short bouts of I-want-to-run-away-to-the-hills, there has been rain and high wind and storms. Big, violent storms. And rain at times it usually does not do so in Kolkata. And the almost constant presence of clouds has made the sunsets absolutely gorgeous.

Rainbows

…the rainbow necessarily has three colours, and these three and no others.

It is not difficult to imagine that, during these spells of rain, there might come instances, short periods of time when, looking up towards the heavens, one would see that slate-grey rain clouds covering half the firmament, delivering their watery loads to the thirsty patches below, while on the other end, the sun, having peaked out tentatively from its nebular veil, would shine gloriously for an instant, showering its silver rays through the curtain of rain water.

Essentially, one expects rainbows.

Photograph of a double rainbow appearing over the 18th hole during the third round of the Utah Championship on July 13, 2013 in Sandy, Utah. Photo Credit : Stan Badz/PGA TOUR.
A double rainbow appears over the 18th hole during the third round of the Utah Championship on July 13, 2013 in Sandy, Utah. Photo Credit : Stan Badz/PGA TOUR.

Rainbows are a staple of cultures throughout the geography and history of the world. In Estonian, for instance, the rainbow is the bow wielded by the thunder-god Erruk. In the Nordic Eddas, and now in the Marvel films, the rainbow comes from the Asgardian Bifrost, the bridge of many colours. In Bangla, the word is Ramdhonu, or Ram’s Bow. In Hindi, this transforms to Indradhanu, Indra’s Bow. Perhaps unsurprisingly, Indra is the thunder-god in the Hindu pantheon.

The Greeks

Clearly, the ancients realised the correlation, if not the causation, behind rainbows and unsettled weather. Aristotle, the great Greek natural philosopher, was perhaps the first to peer closer to the rainbow in an attempt to classify the colours within, perhaps in a hope to divining its nature and purpose. Aristotle attempted to reconcile the colours of the rainbow with his theory that all colours came from white and black. In his book Meterologica, he says

…the rainbow necessarily has three colours, and these three and no others.

Bust of Aristotle. Marble, Roman copy after a Greek bronze original by Lysippos from 330 BC; the alabaster mantle is a modern addition. Caption text from Wikipedia. Image Credit : Ludovisi Collection. Image Photographer : Jastrow.
Bust of Aristotle. Marble, Roman copy after a Greek bronze original by Lysippos from 330 BC; the alabaster mantle is a modern addition. Caption text from Wikipedia. Image Credit : Ludovisi Collection. Image Photographer : Jastrow.

Aristotle’s triad of rainbow colours is the same as that suggested by his predecessor, Xenophanes of Colophon. These are porphyra (dark purple), khloros (green), and erythros (red). Aristotle allows for a fourth colour, yellow, a non-primary bright colour that is darker than white but lighter than red, and lives in between red and green.

RGB, rods and cones

From a modern perspective, Aristotle and his predecessor is surprisingly correct. Greek colour names could be a bit…confusing, and porphyra could well be blue. Which means, to them, the three primary colours are blue, green, and red. RGB. All the other colours stem from them. Mix R and G and you have Y, yellow. G + B = C(yan); R + B = M(agenta). White and black, light and dark, add extra dimensions to these colours.

The anatomy of the eye. The retina, that is the screen at the back of the eye, has two types of light-detecting cells. The cylindrical **_rod cells_** detect intensity of light, while the more conical **_cone cells_** detect colour. There are three classes of cone cells, each detecting one of three RGB channels. Image Credit : Unsure.
The anatomy of the eye. The retina, that is the screen at the back of the eye, has two types of light-detecting cells. The cylindrical rod cells detect intensity of light, while the more conical cone cells detect colour. There are three classes of cone cells, each detecting one of three RGB channels. Image Credit : Unsure.

We now know why that is so. Our eyes have three classes of colour detecting cells. Some of these cones are sensitive to red light, some to green light, and others to blue light. The retina also has rod cells; these detect brightness (or darkness). Together, the three cone types and the rods recreate a gamut of colours for human stimulation.

Why, then, do we talk about the seven primary colours? How exactly did VIBGYOR come about?

Newton

Blame Isaac.

Image of a portrait of Isaac Newton (1642-1727). The portrait of Newton is a copy of one painted in 1689 by Sir Godfrey Kneller, which is owned by the 10th Earl of Portsmouth. This copy was painted by Barrington Bramley and donated to the Isaac Newton Institute for Mathematical Sciences in 1992 by the Director of the Institute, Sir Michael Atiyah, who unveiled it at the opening in July of that year. It shows Newton at the height of his scientific acumen, before he went to London to take charge of the Mint.
Portrait of Isaac Newton (1642-1727). The portrait of Newton is a copy of one painted in 1689 by Sir Godfrey Kneller, which is owned by the 10th Earl of Portsmouth. This copy was painted by Barrington Bramley and donated to the Isaac Newton Institute for Mathematical Sciences in 1992 by the Director of the Institute, Sir Michael Atiyah, who unveiled it at the opening in July of that year. It shows Newton at the height of his scientific acumen, before he went to London to take charge of the Mint. Image Credit : Isaac Newton Institute of Mathematical Sciences, Cambridge, UK.

Before Albert, these two words would be oft-heard in the Halls of Physics. Newton was single-handedly responsible for kick-starting and rejuvenating several prime physics disciplines. While he is most well-known for the incident with the apple, his prism comes a close second.

The Original or primary colours are, Red, Yellow, Green, Blew, and a Violet-purple, together with Orange, Indico, and an indefinite variety of Intermediate gradations.

After he was done with gravity in the 1660’s, Newton turned his attention to light. He had read Aristotle and the other Greeks, and was rather motivated by them. Like Aristotle, he too wanted to figure rainbows out, to find out what light is. And he had an analytical tool Aristotle did not; the glass prism.

Newton’s Spectrum

On a bright and sunny day, Newton darkened his room, made a small hole in the window, and placed his prism in front of the hole. Sure enough, on the wall opposite, he saw a beautiful technicolour spectrum. In his own words;

…in order thereto having darkened my chamber, and made a small hole in my window-shuts, to let in a convenient quantity of the Suns light, I placed my Prisme at his entrance, that it might be thereby refracted to the opposite wall. It was at first a very pleasing divertisement, to view the vivid and intense colours produced thereby…

Image of Isaac Newton using a prism to break white light into spectrum. With Cambridge room mate John Wickins. Engraving of 1874.Image credit : Getty Images.
Isaac Newton using a prism to break white light into spectrum. With Cambridge room-mate John Wickins. Engraving of 1874. Image credit : Getty Images.

Once the prism was set, and the spectrum ready, all Newton had to do was to walk over to the wall and mark out the colours of the rainbow. Instead, he asked his assistant (possibly his Cambridge roommate, John Wickins) to do it, remarking later that

because my owne eyes are not very criticall in distinguishing Colours

Not only does this reveal an interesting facet of Newton—that the intensely immensely competitive man did not consider himself perfect–it also shows how English spelling has evolved and changed over the years.

Newton’s Opticks

In 1671-72, Newton published “New Theory about Light and Colours” in the Philosophical Transactions of the Royal Society, in which he reported and explained his results. He wrote (emphasis mine)

There are therefore two sorts of Colours. The one original and simple, the other compounded of these. The Original or primary colours are, Red, Yellow, Green, Blew, and a Violet-purple, together with Orange, Indico, and an indefinite variety of Intermediate gradations.

Image of the cover of Opticks (1704), by Isaac Newton. Image Credit : Isaac Newton / Bibliotheque Nationale de France.
Cover of Opticks (1704), by Isaac Newton. Image Credit : Isaac Newton / Bibliotheque Nationale de France.

More than 30 years later, in his Opticks, he modified his earlier statement slightly

the Spectrum … appear tinged with this Series of Colours, violet, indico, blue, green, yellow, orange, red, together with all their intermediate degrees in a continual succession perpetually varying

including updating the spelling of blue.

Of course, he did more than just label the colours. Careful observations and accurate sketching mixed with a healthy dose of Newtonian analytical genius told him that, as white sunlight passes through a prism, red refracts, that is, changes direction the least, while violet refracts the most. Nowadays we know that light is composed of photons, and that refraction is merely photons decelerating as they pass through an optically denser medium such as glass2. Photons of lower energies, such as those that appear red to us, decelerate the least, while the higher energy violet photons decelerate the most.

Image of Fig. 12, from Newton's Opticks (1704), showing the prism ABC and how the rays refract. G/gamma is the stop, and the sunlight enters through the hole F/phi. Since the sun is so far away, the rays are parallel. Once through the prism, the parallel rays split into components, each parallel to each other. Newton has labelled five of these regions; P/pi (violet), Q/chi (indigo and blue), R/rho (green), S/sigma (yellow and orange), T/tau (red). These fall on the screen _mn_ and form the spectrum. Image Credit : Isaac Newton.
Fig. 12, Opticks (1704), showing the prism ABC and how the rays refract. G/gamma is the stop, and the sunlight enters through the hole F/phi. Since the sun is so far away, the rays are parallel. Once through the prism, the parallel rays split into components, each parallel to each other. Newton has labelled five of these regions; P/pi (violet), Q/chi (indigo and blue), R/rho (green), S/sigma (yellow and orange), T/tau (red). These fall on the screen _mn_ and form the spectrum. Image Credit : Isaac Newton.

Which is all fine and dandy, and proves once again what an absolute magician Isaac Newton was.

But why seven colours?

And we thus come to the rub of the matter. Or the hub of the matter, where lies the rub…

Never mind. This is the central question. Why seven? Why indico, or indigo?

Indigo

Well, first of all, a little bit about indigo. Indigo, as a dye, has ancient origins. According to Pliny the Elder, the Harappans extracted the dye from a certain plant (Indigofera tinctoria) that grew in the Indus valley. The Ancient Greek term for the dye was Ἰνδικὸν φάρμακον (Indikon farmakon). This later became indicum in Latin and later indigo in Portuguese. The Silk Route brought indigo to Europe, when Marco Polo reported about it in 1289. However, a further three centuries went by before the European textile landscape realised the potential of the dye, and started large-scale manufacture. A further three centuries went by before the first artificial dye was invented. Things have rather moved up nowadays, with quantum dots being the latest and best in the colour business.

Photograph of Indigo, historical dye collection of the Technical University of Dresden, Germany. Image Credit : Shisha-Tom.
Indigo, historical dye collection of the Technical University of Dresden, Germany. Image Credit : Shisha-Tom.

Around Newton’s time, clothing dyed in indigo was quite in vogue. The dye itself had an air of exotic orientalism about it, and it might have been quite fashionable to say, “Ah, is that there an indigo doublet that I spy?” instead of saying “I want that blue vest.” Newton was human, though perhaps less so than most, and he must not have been entirely out of step with the times. He must have felt, “I’ll use indico a lot. Indico is cool.”

Cyan

Which perhaps makes sense, except that to some people, it doesn’t. Most of the human race, when asked to identify the colours in a sunlight-prism spectrum, manage to name only six colours. A few sharp-eyed ones might manage seven, but there is a crucial difference. There is no indigo. The colour series is, for most,

Violet > Blue > Cyan > Green > Yellow > Orange > Red

What is cyan, then?

Image of the RGB colour star. The three primary colours--red, blue, green--at the three vertices of the upright triangle. The secondary colours--magenta (R+B), cyan (B+G), yellow (G+R)--at the three vertices of the inverted triangle. Tertiary colours rose (R+M), violet (M+B), azure/teal (B+C), spring green/sea blue (C+G), chartreuse green (G+Y), and orange (Y+R) form the intermediate vertices of the 12-pointed star. Image Credit : DanPMK
RGB colour star. The three primary colours–red, blue, green–at the three vertices of the upright triangle. The secondary colours–magenta (R+B), cyan (B+G), yellow (G+R)–at the three vertices of the inverted triangle. Tertiary colours rose (R+M), violet (M+B), azure/teal (B+C), spring green/sea blue (C+G), chartreuse green (G+Y), and orange (Y+R) form the intermediate vertices of the 12-pointed star. Image Credit : DanPMK

In the RGB colour star above, red, green, and blue are considered primary colours, in tune with human physiology. By mixing two of these primary colours in equal amounts, you get the secondary colours cyan (blue + green), yellow (green + red), and magenta (red + blue). Mix a primary with its adjoining secondary, and you have the tertiary colours. My favourite is obviously chartreuse green, the best colour there is.

Cyan (B+G) is basically what most people refer to as sky-blue. Although, truth be told, looking at the colour star, it kinda looks like azure might be closer to sky-blue. Either way, there is a pretty prominent colour between blue and green on this colour star, and it has non-English analogues as well.

VIBGYOR in Bengal : BayNeeAaShoHoKawLa

I am from Bengal, India. Although we do a lot of English stuff, Bangla is our language. Our first words are in Bangla, and our first connections with this wondrous world is via words in Bangla. And does Bangla have an analogue to VIBGYOR?

Well, of course it does. Although, sadly, urban Bengali kids born after the nineties might not quite have heard of

BayNeeAaShoHoKawLa

In Bangla, বেনিআসহকলা.

I’ll break it down. BayNeeAaShoHoKawLaa3. The colours, and their English equivalents, are

  1. Beguni (baygoonee, বেগুনী): Violet
  2. Neel (kneel, নীল) : Blue
  3. Aakashi (aakaashee, আকাশী)4 : Cyan/Sky-blue/Azure
  4. Shobuj (showbooj, সবুজ) : Green
  5. Holud (howlooð, হলুদ) : Yellow
  6. Komola (kawmohlaa, কমলা) : Orange
  7. Laal (laal, লাল) : Red

Notice any colour missing?

The Modern Spectrum

Image of the modern spectrum. Image credit : Unsure.
Modern spectrum. Image credit : Unsure.

Red photons have the least energy (among visible photons), and so have the longest wavelength. In the image, red appears at around 700 nm. Violet, at the other end of the visible spectrum, has the most energetic photons, and therefore the shortest wavelengths. In the image, violet appears around 400 nm. The human visual range is approximately that, roughly between 400nm to 700 nm. Any lower and we get high energy ultraviolet (UV) rays (< 380 nm); any higher and you have the low energy infrared rays (> 740 nm). Too much of UV is bad and can give you skin cancer; too much of infrared is also bad and can cook you.

Newton’s indico falls between 430 and 450 nm. Looking at the image, to me at least that looks rather blue, or at least blue-violet. I don’t know if you’ll do better. On the contrary, there is a clear cyan band between 475 nm and 500 nm.

Image of prism and spectrum. Image credit : Unsure.
Image of prism and spectrum. Image credit : Unsure.

Here we clearly find a violet, a blue, a cyan, a green, a yellow, and a red. No clear indigo, and, surprisingly, a somewhat muddled orange. Seems BayNeeAaShoHoKawLa works better than VIBGYOR.

Ever since I first learned the sequence of colours in the rainbow, I have been puzzled and annoyed by the inclusion of indigo in this sequence.

Newton wasn’t aware of BayNeeAaShoHoKawLa though, and generations of people after him have sworn by VIBGYOR (or ROYGBIV). No questions were asked till the early part of the 20th century, when people began to analyse indigo and why Newton’s colour spectrum has seven colours. However, the greatest boost probably came in 1973, from a very unlikely source.

Helen Davoll and New Scientist

New Scientist is a popular science magazine5 based in London. Like other magazines, New Scientist too has a Letters section. In the issue dated 13 Dec 1973, among letters from readers providing valuable comments and input on issues as diverse as Jupiter’s Red Spot, methane power, and whether Uri Geller was a charlatan, there was one letter that was, perhaps, a little different. Titled Why Indigo?, the letter begins with these words :

Ever since I first learned the sequence of colours in the rainbow, I have been puzzled and annoyed by the inclusion of indigo in this sequence.

A sentiment, I’m sure, most of us share.

Image of the cover of New Scientist 13 December 1973. Image Credit : New Scientist.
Image of the cover of New Scientist 13 December 1973. Image Credit : New Scientist.

The letter was sent by a Mrs H Davoll, of 10 Broadlands Avenue, Shepperton, Middlesex, UK. She goes on to state that

I feel that there is an element of the Emperor’s clothes situation in this matter, most people not daring to admit that they cannot distinguish another colour between blue and violet.

Once again, I for one am completely in agreement with her.

This letter was followed by no less than five replies from various readers, and was preceded and succeeded by a number of printed peer-reviewed publications analysing the same question Mrs Davoll had asked, Why Indigo?

Next on VIBGYOR

In the next installment of this series on VIBGYOR, The Nerd Druid shall investigate these explanations. The Nerd Druid shall also attempt to uncover who Mrs Davoll was, and will trace the sequence of comments and letters in the New Scientist. Until then, we should remember that this June…

Pride

…is Pride month in the US. For the LGBTQ community, it is a time to come out in droves and celebrate life as normal human beings, to stand out from the stigma and oppression that accompanies them. It is a time for them to appreciate the full spectrum of which conspicuously omits indigo life. Which is why, appropriately, their symbols is the rainbow flag (six colours, no indigo):

Rainbow Pride Flag. The Pride Flag is the symbol of the LGBTQ community. It has six colours. Unsurprisingly, there is no indigo. Image Credit : [Ludovic Berton](https://www.flickr.com/people/23912576@N05)

References

Books

  1. Aristotle : Meterology, Greece (350 BCE).
  2. Newton, Isaac : Opticks or, a Treatise of the reflexions, refractions, inflexions and colours of light . Also two treatises of the species and magnitude of curvilinear figures, Sam Smith & Benj. Walford, for the Royal Society (MDCCIV, 1704).

Papers

  1. Newton, Isaac : New Theory about Light and Colours, Philosophical Transactions (1672).
  2. McLaren, K. : Newton’s Indigo, Color Research and Application (1985).
Facebook Comments

Footnotes

  1. Or, as modern audiences call it, a Game of Thrones.
  2. Remember that acceleration (or deceleration) is change of velocity, a vector quantity. You can change velocity by either changing the speed or the direction of motion.
  3. Does this remind you of the music scale? You know, the Sound of Music one, Do-Re-Mi? Yes? No? Well, I’ll come to that in the next part of this series.
  4. People in Bangladesh call this Aasmani (aasmaanee, আসমানি)
  5. Both, popular science magazine as well as popular science magazine. It is a magazine of popular science, and it is a popular magazine.

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