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?