Why can we see so many different colours, and how can there be colours that are not in the light spectrum?
Colour is all around us, we see it in every rainbow, every bouquet of flowers and every beautiful painting. But what exactly is colour – and why are there so many colours?
The reason for the rainbow
When a beam of light enters our eyes, it stimulates cells in the retina – the inside wall of the eye. These super-sensitive cells then produce a signal that our brain interprets as colour. Different types of light produce different signals, and therefore our brain interprets them as different colours.
To understand more about how we see such a variety of colours, let’s consider the different types of these photoreceptor cells we find in the retina. There are two main types – called rods and cones. (Remarkably enough, rods are rod-shaped and cones are cone-shaped.) Rods let us see in the night time. They are sensitive but only detect the brightness of light, meaning they give us black and white vision at night. If we only had rod cells – such as those with the condition achromatopsia – we would only see the world in black and white.
Cones, however, let us see colours. We have three different types of cone, each specialised for a different wavelength (or energy) of light – red, green and blue.
It is the millions of specific combinations of signals from these three cones that allow us to see so many different colours. The reason that some people (and some animals, e.g. dogs) are colour blind is because they lack one or more of these three types of cones, limiting the range of light wavelengths their eyes can detect, and therefore the number of colours they see.
(Interestingly if you rub your eyes really hard, you can put enough pressure on your eyes to stimulate the cone cells – and you will see psychedelic colours. Go easy though, don’t hurt yourself.)
Beyond the visible spectrum and why pink isn’t in the rainbow
The spectrum of light doesn’t stop at the limits of the visible rainbow, however (see below). Red is at the bottom of our light spectrum because it is the lowest energy of light that our eyes can detect. Anything just below the limits of our cone cells is called infrared. And light that is too energetic for our blue cone cells is ultraviolet.
If you don’t believe that there is light beyond what you can see, take a photo of a TV remote with your smartphone. Point the remote at the camera and take a snap while pushing a button on the remote – you will ‘see’ the infrared beam coming from the end of the remote. This is because digital cameras are more sensitive than the human eye and so can pick up more wavelengths of light than we can.
There are also colours that our brain ‘sees’, but that don’t exist in the light spectrum. Pink doesn’t exist in the rainbow – it is how our brain makes sense of seeing blue and red (at the opposite ends of the light spectrum) together. (Watch this video for more.)
But what if you could see more of the light spectrum? Some creatures, such as bees, can see high-energy ultra-violet light. Flowers take advantage of this and have attractive ultraviolet patterns to attract insect visitors – patterns invisible to you or I (see right).
Amazingly, it is not just exotic animals that can have this super-vision. Some humans are born with a fourth type of cone, a condition called tetrachromacy. This literally adds another dimension to their perception of colour, allowing tetrachromats to see millions more colours than the average person.
Answer by Nick Waszkowycz (with Dr Stu and Kyle Pastor). Question from ‘Jay BeBee’ via Facebook.
Image credit: Carmelo Speltino on flickr