These are the colours your eyes can’t see
From an early age, we’re lead to believe that the so-called colour spectrum contains all the colours out there. And it’s true, if you are only considering colours that are perceptible by a normal human eye.
Colour is the limited range of light along the electromagnetic spectrum that humans can see. In other words, colours are the way your brain, and mine, by use of our eyes, interpret a stream of minute energy packets radiating at varying frequencies in a wave motion.
So, if we’re only able to interpret a limited range of light, does that mean there are colours we can’t see? Well, there is such a thing called “forbidden” or “impossible” colours that some believe exist, but that we can’t see due to something called the opponent process.
The opponent-process describes how the light frequencies of colours, which are composed of pairs of hues, automatically cancel each other out in the human eye. The process suggests it is not so much that the colours can never be seen, but rather they are imperceptible due to a limitation in the way the human eye sees colour in the first place.
Many vision researchers criticise the possibility of impossible colours, stating they’re just intermediary colours between two others. But experiments have been performed in which colours have been witnessed that are typically not available to the human eye.
Impossible colours may not exist as individual colours like red, blue, and green, but more likely is that the brain can be “tricked” into seeing something unfamiliar and to appreciate a wider spectrum of colour than we thought was possible.
But first things first… how do we even see colour in the first place?
How do we see colour?
We classify different types of light based on their wavelength—which can be anything from many metres in between each wave peak to as short as the diameter of an atom.
The full electromagnetic spectrum ranges from high energy light waves with short wavelengths, like cosmic and gamma rays, to low energy light waves with long wavelengths, like radio waves.
The light we are able to see, called the visible light spectrum, ranges from violet light, which has a wavelength of around 400 nanometres, to red light, which has a wavelength of around 650-700 nanometres.
When light bounces off an object and into our eyes, it first hits the cornea, the transparent outer layer. The cornea then bends light towards the pupil, which widens or constricts to let in more or less light. The light continues to the lens, which angles and focuses it to a point on the outer layer of the retina. Once the light is detected by the cells in the back of the retina, the information is sent along the optic nerve to the brain, where the signals are processed and interpreted.
The retina is made up of light-sensitive cells called rods and cones that send information to nerve cells in the inner retina. Rods are responsible for our perception of light and dark and our peripheral vision. Cones, found in the centre of the retina, are what allow us to see colour. They come in three kinds, each sensitive to a different range of lightwaves.
We have more cones sensitive to red light than any other type, meaning our vision is best for warmer colours such as reds, oranges, and yellows. But our cones are limited to perceiving lightwaves that fall within the range of 400 nanometres and 700 nanometres.
A gamma wave’s wavelength, for example, is about the length of a nucleus of an atom, which is much to short for our cones to sense. Radiowaves are too long, with waves the length of two empire state buildings put together.
Other animals have different kinds and amounts of cones which allow them to see more or less of the electromagnetic spectrum. Dogs, for example, are more limited in the wavelengths they can see, but butterflies can see ultraviolet light. However, the winning prize goes to the mantis shrimp, which has twelve different kinds of cones.
The impossible Colors
So, now we understand a little more about the human eye works and how we see colour, we can start to get a better idea about these so-called impossible colours.
As we know, the human eye has three types of cones that allow us to see a certain range of light, and, therefore, colour, on the electromagnetic spectrum—i.e., the visible light spectrum. These colours are blue, green, and red.
Of course, we see much more colour than just these three. There is an overlap between the wavelengths of light covered by the cone cells. White, for example, is not a wavelength of light, yet we perceive it as a mix of different colours. Now, due to antagonistic fashion in which colours work and the opponent process, we can’t see certain colours at the same time, i.e. blue versus yellow, red versus green, and light versus dark.
The colours blueish-yellow and greenish-red are the alleged “impossible” colours we can’t see.
Is it possible to see impossible colours?
The opponent-process theory of colour perception has been around since the 1970s and basically states that it is impossible for any human to perceive colours such as blueish-yellow and greenish-red. However, in the 1980s, two researchers conducted an experiment in order to see if it was possible to “trick” the brain into seeing these colours.
Their experiment—not recommended at home—involved subjects staring at an image made up of two strips that were coloured red and green. To ensure the subject’s eyes remained fixed on the opposing colours, their heads were stabilised with a chin rest and the eye movements were tracked using a camera.
On top of this, the images moved with their eye movements, so the individuals received a continuous wavelength of light. The results of the experiment were surprising, even to the researchers. In the right conditions and for the right amount of time, the border between the opposing colours seemed to gradually dissolve, and the so-called forbidden or impossible colours emerged.
Human eyes, with their over two million working parts, allow us to see a vast range of colour. But it is not the only way to see the world. Maybe a day in the life of a mantis shrimp would open our eyes to other ways of seeing, and just how colourful the world can be.
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