These are the Colours Your Eyes Can’t See

It is quite common that we humans assume the so-called colour spectrum contains all the colours that exist in the world. And that’s true – that is, all the colours that exist in the human world. In reality, there are lots of colours that are simply not perceptible by the average human eye.

For the sake of most arguments, colour is simply considered the range of light across the electromagnetic spectrum that humans can see. In other words, colours are essentially the way our brains – with help from our eyes – interpret a stream of minute energy packets radiating at varying frequencies in a wave mention. But let’s be honest – that’s a bit of a mouthful.

What we really need to know is that our eyes, in all their complexity, can only interpret a limited range of light. So, that means that there are colours that we can’t see due to a phenomenon known as the opponent process. These are known as “forbidden” or “impossible” colours.

The Opponent Process

The “opponent process” theory states that the colours we see are paired into opposing hues. When we perceive these opposing colours (e.g. red-green, blue-yellow, black-white) they automatically cancel each other out. This process suggests that this limitation in the way the human eye sees colour means that some colours are simply imperceptible.

However, some vision researchers argue that these so-called “impossible” colours are simply just intermediary colours between two others. Nonetheless, some experiments have demonstrated that some colours have been seen that are not typically available to the human eye.

Impossible colours may not exist as individual colours like red, blue, and green, but various studies have now demonstrated 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?

Our eyes classify light based on its wavelength – which can vary from many metres between every ‘wave’ peak to as little 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 –  known as 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.

How the eye works

Our vision works by picking up light that bounces off objects around us. This light enters our eyes through the cornea (the transparent outer layer) and pupil at the very front of the eye. The cornea bends the light into the eye while the pupil widens or constricts to let in more or less light.

From here, the refracted light hits the lens inside, which angles and focuses it onto the retina at the back of the eye. The light is detected by special cells in the back of the retina and information is sent along the optic nerve to the brain where the image is processed and interpreted.

The retina is made up of light-sensitive cells called rods and cones. These cells send information to the nerve cells in the inner retina: rods are responsible for our perception of light and dark and our peripheral vision; cones are what allow us to perceive colour.

There are three types of cones, each of which is sensitive to a different range of light waves. We have more cones that are sensitive to red light than any other type, which means that our vision is best suited for warmer colours such as reds, yellows, and oranges. But our cones are limited to perceiving lightwaves that fall within the range of 400 nanometres and 700 nanometres.

For example, the wavelength of a gamma ray is about the length of the nucleus of an atom – which is much too short for our cones to pick up. At the other end of the spectrum, radio waves are too long at the length of two empire state buildings stacked one on top of the other!

As you might expect, other animals have different kinds of cones which allow them to see a wider or lesser range of the electromagnetic spectrum. Dogs, for example, are more limited in the wavelengths – and hence, colours – they can see, while butterflies can see ultraviolet light that humans are unable to detect. The prize for most wide-ranging vision, however, goes to the mantis shrimp, which has 12 different kinds of cones.

The impossible Colours

Now that we have a clearer idea of how the human eye works to detect colour, we can start to understand a little more about these so-called “impossible” colours.

As we mentioned above, 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.

This experiment – which it is not recommended you try at home – involved subjects staring at an image made up of two strips: one red and one green. The subjects’ heads were stabilised with a chin rest and their eye movements were tracked using a camera. The images moved with their eye movements to ensure their eyes remained fixed on the opposing colours and they 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.

Our eyes, with their over two million working parts, allow us to see a vast range of colours – but it isn’t the only way to see the world.  Perhaps 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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