Monthly Archives: June 2009

The rays are not coloured

So when Newton wrote that the rays are not coloured, what exactly did he mean?

Well, he meant that even though we may say loosely that light at 400nm is blue and light at 700nm is red this implies that the blueness and the redness are properties of light.  Although there are philosophical arguments that would support colour as a property of light (and we’ll get on to those arguments in a later post) for now I would like to put forward my view (which is, I believe, consistent with Newton’s) that colour is not the property of light.

The evidence that supports my view is that light at 700nm may look red to most people most of the time, it doesn’t look red to all of the people most of the time or even to most of the people all of the time. For a very striking example please consider the image below:


In this example, you will see some blue spirals and some green spirals. But physically the blue and green are the same. In terms of wavelengths, exactly the same wavelengths (in exactly the same proportions) are being reflected from the areas that you perceive as being green and the those you perceive as being blue. If you think in terms of digital (RGB) terms, the RGB values of the green areas and the blue areas are the same – both are about R = 9, G = 20, B = 160. We know now that the colour that you perceive for a wavelength of light or a group of wavelengths depends upon the colours that are close by. This is often expressed as contrast or assimilation. When contrast occurs colours become less like the colours that they are next to an image; when assimilation occurs colours become more like the colours that they are next to. Contrast and assimilation effects result in you seeing two colours, a blue and a green, when physically only one colour exists.

Straight away some of you can see that I am falling into loose language straight away because I am using colour in two different ways. On the one hand I am saying the two colours are physically the same and on the other hand I am saying that the two colours are perceptually different (blue and green). Which is it? It all depends upon how you define colour. My stance is that I define colour as a perceptual phenomonon – it’s something we see and experience. Others may argue that the two colours are really the same and that it is a mere illusion that they look different – I, on the other hand, would argue that the two colours are different. It’s not an illusion – you see a blue and a green, don’t you?

This is what Newton was referring to when he said that “to speak properly, the rays are not coloured” – I believe that Newton was aware of this problem with language – that colour can be used to represent several things. But when we speak properly we realise that the rays are not coloured.

Colour 101

I am really looking forward to some interesting topics such as

Is black a colour?


Does colour exist?

But, before I get into these tough topics I would like to present some basic and rudimentary notions about colour and what it is. Look in any textbook on colour and you’re almost certain to find a picture of the electromagnetic spectrum looking something like this:



It was Newton, of course, who famously studied the relationship between wavelength and colour. Light is a form of energy called electromagnetic radiation. Light can be characterised by its wavelength and our visual systems are sensitive to wavelengths in the approximate range 400-700nm (we’ll deal with the exact wavelength range later). So we call radiation in this range the visible spectrum or, more simply, light. In my diagram above the short wavelengths are on the right and the longer wavelengths on the left. So we might simplistically think that, for example, light at 400nm is blue or violet and that light at 700nm is red. It’s nowhere near as simple as this but it would do no harm to think that way for the present.

The spectrum above raises two interesting questions straight away however. The first is, why – since the wavelength of light varies continuously from about 400nm to about 700nm – do we see these specific and discrete colours? When I was at school I learned the mnenomic Richard Of York Gave Battle in Vain to remember the order of the colours in the spectrum. But why don’t we see a continuous range of colours – or, to be technically more precise – hues? The answer is something called categorical perception. However, just as interesting is my second question. Why do the two ends of the spectrum look rather similar. OK, red and violet are not the same. But certainly, red is closer to violet perceptually than it is, to say, green. And yet in wavelength terms red is closer to green! I’ll be returning to this issue of circularity of hue in a later post. However, if you would like to explore either of these phenomena yourself then I would encourage you to spend time looking at a rainbow. When sunlight strikes droplets of water in the air (this often happens on a sunny day after a rainstorm) the wavelengths separate (a process called refraction) and we see the visible spectrum. Newton achieved this by passing sunlight through a glass prism but the effect is the same, and equally enjoyable.

Interestingly, although Newton observed 7 colours when he separated white light with his glass prism, most scientists today agree that it is really only possible to discern 6 colours and that indigo cannot be distinguished from violet in the visible spectrum. Again, don’t take my word for it. Go out and look a rainbow now!!! The following relationships between colour and wavelength are often quoted:

Red —- 635-700nm

Orange —- 590-635nm

Yellow —- 560-590nm

Green —- 490-560nm

Blue —- 450-490nm

Violet —- 400-450nm

However, be very careful. Newton famously wrote that “to speak properly, the rays are not coloured”. Now, I wonder what he could have meant by that?

what this blog is about

Well …. Hello.

It’s taken my a while to dip my toes into blogging. The thing that has finally made me do it is my recent (positive) experience with facebook and twitter. The way in which these different entities are starting to interact has really caught my imagination and I can really see how blogging can enrichen that interaction further.

So, what have I got to say and why should anyone read it?

The topic of this blog is going to be colour. I am fascinated by colour and have been working in this area for about a quarter of a century. I think I know something about it. I have worked in colour chemistry, colour physics, colour engineering, colour neuroscience, colour psychology, and colour design. I have a PhD in colour and I am a full professor at a major UK University. I am also Head of the School of Design at my university.

Not only am I fascinated by colour but I have a great interest in communicating and educating what I have learned about colour. I have been teaching at university for many years but have also written many papers and books, have written colour software, and have presented colour courses for many companies over the years. Now, I think the time has come to transfer my knowledge and enthusiasm to what I am reliably informed is called the blogosphere.

I hope you find what I have to say interesting, informative, entertaining, and – from time to time – annoying. That’s life – if we all agreed all of the time it would be a very boring place.