Is violet found in the rainbow?

We are taught that Richard Of York Gave Battle In Vain and for this gem we have to thank Isaac Newton who also had a whacky theory of how colour related to music. Whilst we can credit Isaac with some understanding of colour theory, his seven named spectral colours have been grossly misused over the centuries since.

Isaac's colour wheel

And so Johnny is taught in school that the rainbow has seven discrete colours. He is also falsely taught that the primary colours are red, blue and yellow. As a child I quickly learned from experiment the falsehood here. You try getting a decent purple by mixing red and blue paints! It would be more correct to say that the primary colours for additive mixing are red, blue and green (but c.f.) and that the colours you get from mixing these in equal proportions, aka secondary colours, namely yellow, cyan and magenta, are for this very reason the primary colours for subtractive (paint) mixing.

In our school room here we have a colourful set of of containers which I have pictured below, Which of the container colours shown are not found in the spectrum of white light?

A friend JHA recently gave an dissertation on blue and red, as in the colours of the rainbow. He excluded purple and did not mention violet thus implying that the ends of the visible spectrum are red and blue. I consider this a gross injustice, I feel as though I have been robbed. Whilst I can do away with indigo because the truth is that the spectrum is a continuous variation in colour, to say the rainbow ends in blue is a blatant lie. But the truth of the matter is not so clear. I thought JHA would go on to talk about green as the third "primary colour" and thus disprove the law of the excluded middle but the point he was making was somewhat different. But whatever he was talking about, he very definitely did not mention spectral violet.

So what is the truth? All sources agree that human eyes are trichromatic and thus possess three types of colour sensor or "cones" which have peak spectral responses around blue, green and red, and this is why colour TV and film can reproduce most colours using pigments of these colours aka primary colours. Although some folk, 10% of men I am told, only have two types of cone and are thus "colour blind". There is even the suggestion that some women might even have a forth type of cone which I regard as wholly unfair.  So far so good.  But hereafter theories differ.

There are those who deny that there is a problem at all. Until I hear him recant I must place my friend JHA in this category. For such people violet cannot exist in the rainbow.

Some say that the the "blue" cone is a misnomer because it actually has peak response to what we call violet spectral light. Spectral light of slightly longer wavelength actually tickles both the violet cones and to a lesser degree the other cones, and this combination we perceive as blue. And thus the sky is actually violet.

Others (and here) say that in fact the "red" cones have a secondary response in the violet area and thus light of this wavelength tickles both blue and to a lesser degree red cones and we perceive this mixture as violet.

Both parties agree that the eye perceives other mixtures of red and blue as variations of the purple / violet / mauve / magenta colours some of which are rather poorly defined.

In the second and third theories violet is definitely a spectral colour as distinct from blue, so that the only reason scientists might talk about the blue and red ends of the visible spectrum is because blue is an easier concept than violet. After all the first meaning of the word is a small wild flower.

One more (at least!) point that needs to be made is that, even in the rainbow, we are typically not looking at pure spectral colours, by which I imply monochromatic light. You'll get a better sensation of monochromatic light aka pure saturated colour by looking down a spectrometer, a device that splits light into a rainbow of colours e.g. using a prism but uses lenses and a slit to ensure that the eye is looking as only a very narrow angle of the emitted spectrum. As most of us do not have access to a spectrometer, try looking directly at a prism splitting white light. A chandelier crystal will do.

Colour film and TV can never do full justice to the eye's ability to see colour. Indeed the eye itself can never do full justice to the colours actually implicit in the spectrum. In either case the issue is firstly that the eye is trichromatic and secondly that the red, green and blue cones each respond to a broad range of colours and thus, apart possibly from the extreme ends of the visible spectrum, any particular spectral colour is bound to stimulate more than one type of cone and thus results in reduced saturation. This effect is progressively more evident as you move away from the centre of an oil film spectrum as captured in my recent photo.  Here you will see colours verging on brown, a colour no-one to my knowledge has ever claimed was in the rainbow.

Most treasured possession

When I say that this is my most treasured possession, like saying yellow is my favourite colour, it is somewhat tongue in cheek. But some truth as well. Some might say it should be my wife but I am not at all sure that one "possesses" a wife. Semantics aside, for the purposes of this post I will maintain my ground.

Most treasured possession

What is it? It is a Rank-Strand Cinemoid swatch book, which I have kept all these years in a box that once contained a "2 MEG LIN L/S" potentiometer. Samples of what are known in the trade as "gels" and used to colour stage lights. The price "2/6" dates it before UK decimalisation in 1971. Back then my older sister had a friend who was involved in theatrical lighting and, besotted as I was with all things colour, I begged him to save me some Cinemoid off-cuts. He went beyond all I could have dreamed and acquired this book for me.  Cinemoid was introduced in 1960 - before that theatre lights were coloured by sheets of dyed gelatin hence "gels", but these could easily melt so had a very short lifetime. Cinemoid was made of acetate and was self extinguishing and came in a glorious range of colours. I saved pocket money to buy some sheets, and later was given a pile of off-cuts the remains of which I still have, closely guarded against the perils of community living. Of which, during one notable period, we could own only that which we could fit into our bedroom, and bedrooms back then were quite small and it was not unusual to return from a trip to find that the contents had been moved lock, stock and barrel to another location. And the movers were more interested in getting the job over and done with than with caring for one's possessions. 

with a colour wheel I made

In spite of this restriction I still have my Cinemoid swatch book and still, I am pleased to say, have my wife.

The Cinemoid brand now seems to have been superseded by Rosco and Lee and, I suppose, with RGB LED lamps there is less call for filters. In spite of this Cinemoid swatch books like mine are not so very uncommon and may be got on eBay for about 20 GBP.

I sacrificed parts of some of my swatches to made for example the colour wheel above. When spun this gave a reasonable approximation to "white".

I still love it.

Dated much earlier in Bailey history is the colour wheel in my next picture. My father made this, I suppose, when he was a boy - it is hand-painted with water colours on a card disk about 6" diameter made to fit a Meccano circular plate and the idea was to spin it to demonstrate that white light is composed of a spectrum of colours. From previous experience I know its "white" was not very, in fact more beige. But to prove the point I have mounted the wheel on a small d.c. motor and here you have the results.

My father's colour wheel attempt

Here it is spinning at speed

Slowing down

And finally stationary

For completeness I repeated this experiment using my own colour wheel with somewhat better results. This probably reflects on the better colour purity of Cinemoid filters compared with war time children's water colours!  You'll note also that my father has the traditional seven rainbow colours whereas I have the three primary and three secondary colours. Perhaps this says something about tradition.

My colour wheel attached to d.c. motor

Spinning slowly

Spinning fast the colours mix to a plausible white

Which brings me to my father's secret cupboard and another treasured possession. We children all knew about his cupboard in my parent's bedroom under the steps that led to the attic, attic of model railway fame on one side and my older sister's boudoir on the other. But to open the cupboard - this was strictly out of bounds. Although when suitably sure of not being discovered, I did occasionally peep in. I cannot of course divulge what was in that cupboard apart from to say that he kept his Meccano there. Which Pandora's box I am pleased to have inherited and is shown below.  It was at one time in its more distant past a canteen for cutlery, hence the blue lining inside.

My father's Pandora's box

Treasure inside the box

Smaller parts in the tray

I may have mentioned before the colours on a 56K-ohm resistor which was contributory to my choice of career, colour light signals confirming my love of railways, or the visible spectrum drawing me to loving optics, and so on. And yet if you ask people in the community here I think they would agree that I would be the last person to comment on or contribute to the choice of colour for walls, carpet or curtains. And this is not because I do not have opinions.

As a final reminder that this writer loves colour, I woke this morning to open the curtains (there's another potential blog post hidden in this action) and saw a blue dawn with the yellow moon setting behind the tree line. I did not think my camera would do it justice but the results are not too far from what I actually saw.

A study in blue and yellow