By Liz Elliott

A beam of white light is made up of all the colours in the spectrum. The range extends from red through to violet, with orange, yellow, green and blue in between. But there is one colour that is notable by its absence.

Pink (or magenta, to use its official name) simply isn’t there. But if pink isn’t in the light spectrum, how come we can see it?



Here’s an experiment you can try: stare at the pink circle below for about one minute, then look over at the blank white space next to the image. What do you see? You should see an afterimage. What colour is it?





You should have seen a green afterimage, but why is this significant?

The afterimage always shows the colour that is complementary to the colour of the image. Complementary colours are those that are exact opposites in the way the eye perceives them.

It is a common misconception that red is complementary to green. However, if you try the same experiment as above with a red image, you will see a turquoise afterimage, since red is actually complementary to turquoise. Similarly, orange is complementary to blue, and yellow to violet.



All the colours in the light spectrum have complements that exist within the spectrum – except green. There seems to be some kind of imbalance. What is going on? Is green somehow being discriminated against?



The light spectrum consists of a range of wavelengths of electromagnetic radiation. Red light has the longest wavelength; violet the shortest. The colours in between have wavelengths between those of red and violet light.



When our eyes see colours, they are actually detecting the different wavelengths of the light hitting the retina. Colours are distinguished by their wavelengths, and the brain processes this information and produces a visual display that we experience as colour.



This means that colours only really exist within the brain – light is indeed travelling from objects to our eyes, and each object may well be transmitting/reflecting a different set of wavelengths of light; but what essentially defines a ‘colour’ as opposed to a ‘wavelength’ is created within the brain.



If the eye receives light of more than one wavelength, the colour generated in the brain is formed from the sum of the input responses on the retina. For example, if red light and green light enter the eye at the same time, the resulting colour produced in the brain is yellow, the colour halfway between red and green in the spectrum.



So what does the brain do when our eyes detect wavelengths from both ends of the light spectrum at once (i.e. red and violet light)? Generally speaking, it has two options for interpreting the input data:



a) Sum the input responses to produce a colour halfway between red and violet in the spectrum (which would in this case produce green – not a very representative colour of a red and violet mix)

b) Invent a new colour halfway between red and violet



Magenta is the evidence that the brain takes option b – it has apparently constructed a colour to bridge the gap between red and violet, because such a colour does not exist in the light spectrum. Magenta has no wavelength attributed to it, unlike all the other spectrum colours.



The light spectrum has a colour missing because it does not feel the need to ‘close the loop’ in the way that our brains do. We need colour to make sense of the world, but equally we need to make sense of colour; even if that means taking opposite ends of the spectrum and bringing them together.



Well, now we've got that sorted out, explain this: stare at the dot in the middle of the image below - you should see all the colours melt away.

You can find out more about Liz on her Nullpage.