Category: Optics

Why Indigo? The Mystery behind Newton’s VIBGYOR

Why Indigo? The Mystery behind Newton’s VIBGYOR

Reading Time: 13 minutes

Four and a half centuries ago, Isaac Newton used a prism and a dark room to split white sunlight into its coloured components. He labelled seven primary colours, later known by the mnemonic VIBGYOR. Curiously, he chose indigo to be one of the seven, a colour that most people fail to pick out from the spectrum. Why did Newton choose this elusive colour to be one of his chosen seven? Why indigo? The Nerd Druid Investigates!

VIBGYOR and the rainbow colours

In my previous article, I spoke at length about the history of the colours in the rainbow, what VIBGYOR and its inverse ROYGBIV mean, and how non-anglophone cultures perceive the seven rainbow colours. Here’s a quick recap.

Rainbow in world cultures

Most cultures associate the rainbow with a god holding a bow. For the Estonians, it is the thunder god Erruk. For the Indians, it is either the thunder god Indra or the prince of Ayodhya, Rama. The Norse do not have a bow-wielding divine being. Instead, for them, the rainbow is the Bifröst bridge in divine Asgard.

Entry of the gods into Valhalla via the Bifröst bridge. Nordic mythology. Image Credit : Unknown.
Entry of the gods into Valhalla via the Bifröst bridge. Nordic mythology. Image Credit : Unknown.

Aristotle and the Greeks

The Greeks ascribed three primary colours to the rainbow : porphyra (dark purple), khloros (green), and erythros (red). Aristotle allowed for a fourth, yellow, but made it quite clear that this was a composite, being darker than white but lighter than red.

Newton and the Spectrum

In his 1671-72 paper New Theory about Light and Colours and then, in more detail, in the 1704 masterpiece Opticks, Newton describes how he placed a glass prism in front of a hole in his window, and, having darkened his room, observed on the opposite wall

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…

Newton's diagram of the spectrum of white light due to refraction from a glass prism. Image Credit : Isaac Newton, reproduced in "Memoirs of the life, writings and discoveries of Sir Isaac Newton", by David Brewster.
Newton’s diagram of the spectrum of white light due to refraction from a glass prism. Image Credit : Isaac Newton, reproduced in Memoirs of the life, writings and discoveries of Sir Isaac Newton, by David Brewster.

VIBGYOR and ROYGBIV in anglophone cultures

Post-Newton, anglophone cultures adopted ROYGBIV as a mnemonic for schoolchildren to learn and remember this seried of seven colours. Schoolchildren in the US were told that this refers to a man named Roy G. Biv, while those in the UK learnt a bit of history too, with Richard Of York Gave Battle In Vain.

The inverse, VIBGYOR, which agrees with Newton’s own colour order, is probably an Indian English thing.

BayNeeAaShoHoKawLa, the Bangla rainbow

Bengali people too had a nice mnemonic for the rainbow colours. BayNeeAaShoHoKawLa (বেনীআসহকলা) was, like VIBGYOR, made out of the first letters of the seven colours. However, there is a crucial difference. While VIBGYOR is

Violet > Indigo > Blue > Green > Yellow > Orange > Red

BayNeeAaShoHoKawLa, when translated to its equivalent English colour names, reads

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

Modern spectrum. Image credit : Unsure.
Modern spectrum. Image credit : Unsure.

A quick comparison with a good modern spectrum shows that the Bangla BayNeeAaShoHoKawLa is probably more accurate than VIBGYOR. So, the question is…

Why Indigo?

Most people were perfectly happy with indigo being the second (or the sixth, if you take ROYGBIV) for almost two and a half centuries after Newton’s spectrum. However, in the early part of the 20th century, some curious sould began to ponder the significance of seven primary colours. Some of these theories have long been discredited, others are perhaps more plausible. Analysis of Newton’s choice got a fresh impetus in the 1970s, and one would very much like to believe it was due a letter from a reader in a 1973 edition of the British popular science magazine New Scientist. The letter, by a Mrs H Davoll, of 10 Broadlands Avenue, Shepperton, Middlesex, UK, titled Why Indigo?, was published on 13 Dec 1973. In it, Mrs Davoll says that

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…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.

Let’s find out why, shall we?

Colour nomenclature : Sociopolitical Effects

Compare VIBGYOR with BayNeeAaShoHoKawLa again. I’ll lay them out one after the other, along with a modern spectrum :

Violet > Indigo > Blue > Green > Yellow > Orange > Red

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

Modern spectrum. Image credit : Unsure.
Modern spectrum. Image credit : Unsure.

Let’s imagine, for a moment, that both versions are accurate, and actually refer to the same thing. If that is so, then would we be amiss in thinking that, for some reason, what Newton thought was blue is what we know as cyan nowadays? There are precedents for this sort of linguisting colour labelling phenomenon. Homer, the blind Greek poet of the Epics, referred to

…the wine-dark sea…

while we, social media savvy modern humans, have great difficulties in figuring out the correct colour of an evening gown. Take the colour of the sky, for instance. Some would say it is cyan, some would prefer azure, while people with simpler vocabularies, such as me, would probably have to make do with sky-blue.

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

Newton’s indigo is today’s blue?

Take a look at the RGB colour star. Would you say that the colour of the sky is what is labelled as azure in the diagram? Or is it perhaps closer to cyan? If it were up to me, I would probably opt to lighten azure a little and call it sky-blue.

Therefore, isn’t it at all possible that, just as we posited that Newton’s blue might be today’s cyan, Newton’s indigo might also be today’s blue? After all, indigo had been adopted in Europe as a natural dye since about a hundred years before Newton’s experiment. It is entirely possible Newton himself owned clothing that had been coloured blue using the indigo dye. Would it be impossible for him to possess and wear a blue bowtie, dyed with indigo, and state, “I wear an indigo bowtie now. Indigo bowties are cool!”

Orange and Indigo : Fruit and Dye

Consider oranges. They are a fruit, but they are also a colour. Which came first?

Well, the fruit did. Portuguese merchants brought sweet Indian oranges to Europe in the late 15th century, displacing the bitter Persian oranges grown in southern Europe. The word orange itself has a circuitous route,

narangas (Sanskrit) > narang (Persian) > naranj (Arabic) > naranza > narancia > arancia (Italian) > orange (French, English)

Photograph of an orange and a half. Image Credit : Oasis Botanical Company, unknown photographer.
An orange and a half. Image Credit : Oasis Botanical Company, unknown photographer.

After arriving on English shores, the fruit lent its name to the colour; the first English usage was in 1512. By Newton’s time, a century and a half later, the colour was not just connected to delicious sweet oranges, but had entered daily use, supplanting yellow-red, saffron, and citrine as the most popular word describing that particular colour.

Similarly, indigo, though originally a dye, must have been closely connected with anything dark blue, prompting Newton to use it in his spectrum.

Photograph of a piece of indigo plant dye from India, c. 2.5 inches (6.35 cm) square. Image Credit : Evan Izer (Palladian)
Piece of indigo plant dye from India, c. 2.5 inches (6.35 cm) square. Image Credit : Evan Izer (Palladian)

All this was, of course, quite unnecessary when it came to the other five colours, Newton’s Originals. Red, Yellow, Green, Blue, and Violet were all in use for centuries by the time Newton landed in the scene, and had probably lost their charm as colour names. Perhaps orange and indigo were still exotic enough to be used fashionably.

Why Indigo?

If so, why wasn’t Mrs H Davoll equally bothered about orange? That is because of how current affairs and recent history shaped our perception off colour and usage of certain colour names. Unlike the fruit orange, a daily item in our lives, the dye indigo isn’t as ubiquitous.

The various names for dark blue

For instance, British naval uniforms were first introduced in 1748. They were coloured blue using indigo dye. Soon, though, that particular dark shade of blue came to be known as navy blue. Over the years, indigo has lost out to navy as a label for a dark blue in the fashion world. Among painters, the preferred term for a dark blue pigment is ultramarine.

Painting of Captain Edward Vernon (1723-1794), British Naval officer, wearing a dark blue uniform probably dyed with indigo. Painted in the mid-18th century, oil on canvas. Image Credit : Francis Hayman (painter), Royal Museums Greenwich UK.
Captain Edward Vernon (1723-1794), British Naval officer, wearing a dark blue uniform probably dyed with indigo. Painted in the mid-18th century, oil on canvas. Image Credit : Francis Hayman (painter), Royal Museums Greenwich UK.

Ever since the discovery of the first artificial dye in the mid-19th century, the use of the dye indigo has dropped off steadily, and so has the use of the colour term. While navy blue and ultramarine have survived into the 21st century, indigo has fallen by the wayside. So much so that nowadays, the word is used only when referring to the rainbow, or recollecting Newton’s work.

Colour perception : Psychological Effects

In 1920, Edridge-Green developed a theory of colour perception in which he separated colour vision into seven classes or psycho-physical units. Naturally, these units were based on Newton’s seven colours. According to Edridge-Green, a normal person is hexachromic; she should be able to see six colours–red, orange, yellow, green, blue, and violet. People with extraordinary vision, those who can all seven of Newton’s colours, are classified as heptachromic. These people have eyes and vision sensitive enough to distinguish and detect an indigo in between blue and violet.

It is difficult to believe Newton was a normal hexachromic like most other people; surely one of the greatest human geniuses must have extraordinary vision too, no?

Well, no. In fact, Newton’s vision was actually somewhat poor. So much so that, during his prism spectrum experiment, he had to ask

…an Assistant whose Eyes for distinguishing Colours were more critical than mine…

to draw the boundaries of the spectral colours. In his own words, this was

…because my owne eyes are not very criticall in distinguishing Colours…

Newton’s assistant

Image of Isaac Newton (1642-1727) English scientist and mathematician, using a prism to break white light into spectrum. With Cambridge room mate John Wickins. Engraving of 1874. Image credit : Getty Images.
Isaac Newton (1642-1727) English scientist and mathematician, using a prism to break white light into spectrum. With Cambridge room mate John Wickins. Engraving of 1874. Image credit : Getty Images.

Newton’s assistant in the prism experiment, the heptachromic, was possibly John Wilkins, his roommate at Cambridge. Wilkins and Newton were roommates for 20 years, and possibly enjoyed a sort of Holmes-Watson dynamic. Wilkins left Cambridge in the early 1680s and moved 240 km away to Stoke Edith. There he joined the parish, got married, and had a son. The departure of his Watson hit Newton hard; he buried himself even deeper into his research, and channeled his loneliness into work. In this period, a meeting with the astronomer Edmund Halley led him to channel his energies into what would ultimately become the Principia Mathematica, and with it the birth of calculus.

No, not the Tintin character.

Colour perception : Physical Effects

Raman and bright sunlight

Edridge-Green’s theory in no longer in favour, and has been discredited. Instead, C.V. Raman, in his book The Physiology of Vision (1968) had suggested that perhaps it was Newton’s use of sunlight that led to indigo. Raman’s hypothesis was based on the following fact; use a bright enough source of light for your spectrum experiment and you might well be able to make out a band of colour in between blue and violet, a colour that might as well be called indigo.

Photograph of Chandrasekhara Venkata Raman, Indian physicist. Image Credit : The Nobel Foundation.
Chandrasekhara Venkata Raman, Indian physicist. Image Credit : The Nobel Foundation.

Sounds plausible. However, if you do indeed perform the experiment with sunlight, then, under ideal viewing conditions, you should be able to detect as many as 200 separate hues of colour. Whether you choose to name all of them, or merely a certain subset, would quite likely depend on your personal preferences, shaped by your culture and background.

Thus, I’m afraid we’re still on Why indigo?.

The Glass Prism

Modern optical lenses and prisms are made out of two materials, crown glass and flint glass. The latter contains lead, is denser, and has a higher refractive index, which means light rays bend more while passing through it1 Newton probably used crown glass, although it seems his prism had a slightly higher refractive index than is usual. This was possibly because his prism had some lead in it too. Nevertheless, the spectrum it creates seems heavier on the blue-end.

 

Image showing comparison of Newton's prism spectrum diagram and the modern spectrum. Notice how the green, yellow, orange, and red bands in Newton's diagram are narrower than they should be, while the purple/violet, indigo/blue, and blue/cyan bands are wider. This is possibly because of the kind of glass Newton's prism was constructed out of. Image Credit : (Modern Spectrum) Unknown. (Newton's Spectrum) Isaac Newton. (Composite) The Nerd Druid.
Comparison of Newton’s prism spectrum diagram and the modern spectrum. Notice how the green, yellow, orange, and red bands in Newton’s diagram are narrower than they should be, while the purple/violet, indigo/blue, and blue/cyan bands are wider. This is possibly because of the kind of glass Newton’s prism was constructed out of. Image Credit : (Modern Spectrum) Unknown. (Newton’s Spectrum) Isaac Newton. (Composite) The Nerd Druid.

This bias towards the blue end might have prompted Newton and Wilkins to identify an extra colour in between violet and blue.

Colour number : Mystical Effects

In addition to mathematics and physics, Newton had considerable interest in alchemy, astrology, and theology. He wrote almost two million words on these subjects, a number that is almost twice the million words he wrote on science. His mystical interests have led him to be accused of being

…misled by a predilection for the number seven which during many ages has been regarded with a sort of mystical veneration…

It is entirely possible Newton was as fascinated with the number seven as the ancients were. There were enough references to that number in antiquity for Newton to have been influenced enough to have expanded his original five colours to seven, by inserting orange and indico.

Seven celestial bodies

The number of known celestial bodies in antiquity was seven : the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. Uranus and Neptune were discovered after Newton’s time, and Pluto is now a dwarf planet.

Seven days in a week

There are seven days in a week. Be it the Gregorian or Julian calendars of Europe, or the Bangla and neighbouring calendars of India, we always seem to find seven days. Also, these calendars attribute the days of the week to the seven celestial bodies–Tuesday to Mars, Wednesday to Mercury, Thursday to Jupiter, Friday to Venus, and Saturday to Saturn, the first two being rather obvious.

Image of seven planets, seven days, seven metals, seven gods. Image Credit : Unknown, possibly JoeDubs(?).
Seven planets, seven days, seven metals, seven gods. Image Credit : Unknown, possibly JoeDubs(?).

Seven metals of antiquity

The number of known metals in antiquity was seven–Gold, Silver, Iron, Mercury, Tin, Copper, and Lead. These were associated with the days of the week as well as the planets.

Planets, Days, Metals, Gods

The table below shows the ancient associations.

Metal Day Planet Greek/Roman god Norse god
Gold Sun Sunday Helios/Sol Sunna/Sól
Silver Moon Monday Selene/Luna Máni
Iron Mars Tuesday Ares/Mars Tyr
Mercury Mercury Wednesday Hermes/Mercury Odin
Tin Jupiter Thursday Zeus/Jupiter Thor
Copper Venus Friday Aphrodite/Venus Freya
Lead Saturn Saturday Cronus/Saturn
Animation of planets and days, again. Image Credit : Unknown, possibly JoeDubs(?).
Planets and days, again. Image Credit : Unknown, possibly JoeDubs(?).

Seven Deadly Sins

There are seven deadly sins in Christian theology

  1. Envy
  2. Greed
  3. Pride
  4. Lust
  5. Gluttony
  6. Sloth
  7. Wrath

Of course, one could counteract these evil evilnesses by embodying the seven virtues.

Seven heavens and worlds

Different ancient cultures and religions believed in seven heavens, or worlds divided in parts of seven. For instance, in Hinduism, there are fourteen worlds, and they are divided thus (I’m quoting directly from Wikipedia)

According to some Puranas, the Brahmanda is divided into fourteen worlds. Among these worlds, seven are upper worlds which constitute of Bhuloka (the Earth), Bhuvarloka, Svarloka, Maharloka, Janarloka, Tapoloka and Satyaloka, and seven are lower worlds which constitute of Atala, Vitala, Sutala, Talatala, Mahatala, Rasatala and Patala.

Seven day Creation myth

Creation myths in the Abrahamic religions, particularly Christianity, propound the belief that god built Creation in seven days.

Seven Wonders of the Ancient World

There were Seven Wonders of the Ancient World

  1. Great Pyramid of Giza
  2. Hanging Gardens of Babylon
  3. Temple of Artemis at Ephesus
  4. Statue of Zeus at Olympia
  5. Mausoleum at Halicarnassus
  6. Colossus of Rhodes
  7. Lighthouse of Alexandria.
A collage of The Seven Wonders of the (ancient) world, depicted by 16th-century Dutch artist Maarten van Heemskerck.
A collage of The Seven Wonders of the (ancient) world, depicted by 16th-century Dutch artist Maarten van Heemskerck.

Seven Liberal Arts

In ancient Greece, knowledge of the Seven Liberal Arts were considered essential for a free person. These were

  1. Grammar
  2. Logic
  3. Rhetoric
  4. Arithmetic
  5. Geometry
  6. Theory of music
  7. Astronomy.

Correspondingly, or perhaps not, the number of core subjects one has to write one’s tenth level exam in Bengal, the dreaded Madhyamik, is also seven. Bengali and English are the languages, History and Geography make up the humanities, while Mathematics, Physical Sciences (Phys+Chem) and Life Sciences (Biology) make up the sciences.

However inspired Newton might have been from these many examples to expand his colour roster to seven, one should remember that he was, after all, a consummate logical scientist. Would he really have no scientific reason for inserting indigo and orange? Shouldn’t one justifiably expect scientific answers to why indigo, as also why orange? Answers that are not only scientific, but also reveal a crucial insight about how the world works?

Colour pitch : Light and Music

DO – RE – MI – FA – SOL – LA – TI

Julie Andrews in the 1964 Hollywood movie</em> The Sound of Music. <em>The most famous song in this musical, Doe-a-deer, teaches the Do-Re-Mi scale. Image Credit : sound-of-music-dot-com.
Julie Andrews in the 1964 Hollywood movie The Sound of Music. The most famous song in this musical, Doe-a-deer, teaches the Do-Re-Mi scale. Image Credit : sound-of-music-dot-com.

If you have seen The Sound of Music, you will be familiar with these seven notes. Sung joyously by Julie Andrews, these seven are the names of the notes in the diatonic scale. These can also be represented as

C – D – E – F – G – A – B

or as

Sa – Re – Ga – Ma – Pa -Dha – Ni

in Hindustani classical music.

Musical notes are audio frequencies

Musical notes are, essentially, audio frequencies. In Western music, the A key in the fourth CDEFGAB series of the piano, denoted as A4, is set to exactly 440 Hz, and other notes are derived from it by changing frequency in units called semitones. Each octave has 12 semitone intervals. For instance, C4 is 9 semitones lower than A4. Under the popular equal temperament scheme, this makes C4‘s frequency 261.63 Hz.

A larger unit in music theory is the whole tone. It is double the semitone, and together, these two are instrumental in setting scales. For instance, in a diatonic scale of seven notes, there are 5 whole tones (T) and 2 semitones (S), always in groups of TTS and TTTS. Where you start from determines which mode you are following.

Ionian Mode

The CDEFGAB series that I began this section with is in the oft-used Ionian mode, with intervals TTSTTTS. Thus, in order to go from C to D, or from D to E, you need to increase by a whole tone T. E to F is a semitone S. F to G, G to A, and A to B are three whole tone (T) intervals TTT. Finally, to get to the higher C, you need to increase by a semitone (S).

Dorian Mode

In Newton’s time, however, the Dorian mode was much more in vogue, with notes DEFGABCD and intervals TSTTTST. The table below might help

Mode Ionian Dorian
Notes C–D–E–F–G–A–B–C D–E–F–G–A–B–C–D
Intervals T–T–S–T–T–T–S T–S–T–T–T–S–T

In the Dorian mode, the semitones occur at positions 2 and 6, exactly at the positions orange and indigo appear. Adding Newton to the table above (and removing Ionian mode), we get the answer to why indigo.

Mode Dorian Newtonian
Notes D–E–F–G–A–B–C–D R-O-Y-G-B-I-V
Intervals T–S–T–T–T–S–T P-S-P-P-P-S-P

where P are primary colours and S secondary.

 

Figure 4 (pg 91) from Newton's _Opticks_ (1704), slightly cropped. I have added the colours, the notes, and the Dorian intervals. Answers the question : Why indigo? Image Credit : Isaac Newton / The Nerd Druid.
Figure 4 (pg 91) from Newton’s _Opticks_ (1704), slightly cropped. I have added the colours, the notes, and the Dorian intervals. Answers the question : Why indigo? Image Credit : Isaac Newton / The Nerd Druid.

Newton’s Insight

As Newton himself writes (Opticks, pg 92)

…I delineated therefore in a Paper the perimeter of the Spectrum FAPGMT, and … I found that the … rectilinear sides MG and FA were by the said cross lines divided after the manner of a musical Chord…to be in proportion to one another, as the numbers, 1, 8/9, 5/6, 3/4, 2/3, 3/5, 9/16, 1/2, and so to represent the Chords of the Key, and of a Tone, a third Minor, a fourth, a fifth, a sixth Major, a seventh, and an eighth above that Key: And the intervals Mα, αγ, γε, εη, ηι, ιλ, and λG, will be the spaces which the several Colours (red, orange, yellow, green, blue, indico, violet) take up.

Appreciate Newton’s insight and genius. What are colours? They are simply frequencies of light; their pitch are seen and not heard, their instruments of detection are the eyes and not the ears. They are both, ultimately, waves. Very different types of waves, granted, but waves nevertheless, with frequencies and wavelengths.

Why indigo? Because Music!

Image of three centuries of colour scales, beginning with the pioneer, Isaac Newton, who connected DEFGABCD with ROYGBIVR. Image Credit : Unknown.
Three centuries of colour scales, beginning with the pioneer, Isaac Newton, who connected DEFGABCD to ROYGBIVR. Image Credit : Unknown.

 


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 and references therein, Color Research and Application (1985).

Articles

  1. Fisher, Len : Perceptual thresholds: Music inspired Newton’s rainbow, Nature (2015).
  2. Morr, Kelly : Why are there 7 colors in the rainbow?, 99designs, (2016).
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).