Tag Archives: rainbow

The Wizard of Oz

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This week I had to mark about 50 essays that had been submitted for the Colour: Art and Science module I teach at the University of Leeds. One essay looks rather like another after the first 10 or so. So it was a delight to discover that one student had decided to focus on a movie – The Wizard of Oz – and demonstrate her understanding of colour by analysing this classic movie.

It reminded me of a story my mother told me. When she went to see the Wizard of Oz in the cinema (she would have been about 8 at the time) she had never seen a colour movie before. She was so much looking forward to this new-fangled and exciting technology. It’s hard to imagine how exciting that would have been – if every movie you had ever seen had been in black and white!!

Well, imagine her disappointment when the movie started and the movie was black and white after all. For those who don’t know, the movie starts off in black and white (in the Kansas scenes) and only turns coloured when Dorothy is whisked off by the tornado and dropped off in the land of Oz. It must have been a wonderful moment when the screen just turned full colour!!

why is hue circular?

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Everyone is familiar with the colour spectrum. If you pass white sunlight through a prism then it splits into the component wavelengths. The shorter wavelengths appear blue, the longer wavelengths appear red, and in between we have the familiar colours that I learned as school as Richard Of York Gave Battle In Vain, for the sequence red, orange, yellow, green, blue, indigo and violet, and that I have since understood is taught in the US as a person: Roy G Biv. I wonder if there are any other mnemonics that people know of? Of course, many people believe that Newton was in error when he identified 7 colours in the spectrum – he was probably influenced by Aristotle who wrote about there being 7 fundamental colours as there are 7 tones in the musical octave. I’ve posted about the indigo issue before – http://colourware.wordpress.com/2009/07/20/indigo-a-colour-of-the-rainbow/ – so won’t repeat that here.

Newton was probably the first person to create a hue circle (others, such as Forsius, created colour cicles but often included white and black in the circles). Newton created a true hue cirlce where he took the colour spectrum and wrapped it around, noticing that the two ends of the spectrum (where the reds become bluish and the blues become reddish) look rather similar.

Of course, there was a gap because the two ends of the spectrum did not quite match and thus Newton had to add in some purplish colours – these are hues that are never seen in the spectrum (and are sometimes called extra-spectral hues or non-spectral hues). The hues in the spectrum can be created by a single wavelength; however, the extra-spectral hues only occur when we see several wavelengths at the same time. For example, when we see short and long wavelengths together we can see purple.

In my lecture at the University of Leeds (www.leeds.ac.uk) this week someone asked “Why do the two ends of the spectrum look similar at all when the light is so different physically (at one end the waves are short and high energy and at the other they are long and low energy)?” Very very good question – if changes in wavelengths change the hue why should wavelengths that are so different look so similar?

So, why is hue circular? The answer is that it has very little to do with wavelengths and physics and more to do with human physiology. The human visual system captures light with three classes of cell (called cones) in the retinae of the eye. The signals from these cones are processed by the human visual system to create opponent signals (red-green and yellow-blue). This puts red and green opposite each other and yellow blue opposite each other and results in the perception of hue being circular. It also explains why some hues particularly contrast – sometimes called complementary colour harmony.

Indigo – a colour of the rainbow?

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From time to time I come across web pages and groups of people who get irrate about indigo being in the rainbow. There is even a facebook group called “Get Indigo out of the rainbow”. It was Newton who suggested that the rainbow contains seven colours: red, orange, yellow, green, blue, indigo and violet. It has been suggested that, at the time, Newton was trying make some anology with the musical scale and the octave (with its seven intervals) and hence was keen to identify seven colours in the rainbow or visible spectrum. Many modern commentators claim that only six distinct colours can be observed in the rainbow.

Interestingly, the facebook group referred to above would like to eject indigo from the spectrum on the basis that it is not a primary or secondary colour but rather a tertiary colour. The group shows the following colour wheel:

colour wheel

In this so-called painters’ wheel the primary colours are red, yellow and blue and the secondary colours are orange, green and violet. It is argued that since six of the colours in the rainbow are primary or secondary colours in the colour wheel and indigo is not, then indigo has no right to be there. This is wrong on so many levels it is hard to know where to start.

The first thing I would have to say is that this argument seems to ignore the difference between additive and subtractive mixing. Additive mixing – http://colourware.wordpress.com/2009/07/13/additive-colour-mixing/ – describes how light is mixed and the additive primaries are red, green and blue. The additive secondaries are cyan, magenta and yellow. Orange is not in sight – and yet surely if we are to make an argument for inclusion in the spectrum based on primaries (and/or secondaries) then it is the additive system that we should be using since the spectrum is emitted light.  

The optimal subtractive system primaries are cyan, magenta and yellow (with the secondaries being red, green and blue) though the artists’ colour wheel (which is like the painters’ wheel above) has red, blue and yellow as the primaries. 

In my opinion there is nothing special about the colours that we see in the spectrum. Indeed, orange is clearly a mixture of red and yellow and does not seem to me to be a particularly pure colour. I just do not think that arguments to exclude indigo from the spectrum based upon colour wheels or primary colours is valid. That said, I have already mentioned that many people believe that indigo cannot be seen in the spectrum as a separate colour; but this is a phenomenological observation not dogma. I am one of those who believe that indigo and violet cannot be distinguished in the spectrum and therefore I agree with the aims of the facebook group even if I do not agree with their arguments.

The really interesting question is why we see six (or even seven) distinct colour bands in the spectrum when the wavelengths of the spectrum vary smoothly and continuously? I have postulated some possible reasons for this in an earlier post – http://colourware.wordpress.com/2009/07/20/colour-names-affect-consumer-buying/ – but it is far from a complete and convincing explanation. It may explain why we see distinct colours in the rainbow, but why six and why those six in particular. Comments on this would be very very welcome.