The Colour in Your Eye
805 words
Have you ever stared at a bright red shape for thirty seconds, then looked at a white wall? If you have, you might have seen a ghostly green image floating before you. This strange effect is not a trick or an illusion conjured by a magician — it is simply your eyes telling you something remarkable about the way human beings see colour.
Light itself is the starting point for all colour. Sunlight may look white, but it actually contains every colour of the visible spectrum, from red at one end to violet at the other. When light strikes an object, some wavelengths are absorbed and some are reflected back towards your eyes. A ripe tomato looks red because it reflects red wavelengths and absorbs most of the others. A leaf looks green because it reflects green wavelengths. In a completely dark room, without any light to reflect, every object around you would appear the same: invisible.
Once light enters the eye, it travels through the transparent outer layer called the cornea and then through the lens, which focuses it onto a curved surface at the back of the eye called the retina. The retina is only about as thick as a sheet of paper, yet it contains millions of tiny light-detecting cells. These cells do the extraordinary work of turning light into electrical signals that the brain can read.
There are two kinds of light-detecting cells: rods and cones. Rods are extremely sensitive and work well in dim light, but they cannot detect colour — they give you only shades of grey. Cones, on the other hand, are responsible for your rich experience of colour, but they need a reasonable level of brightness to function. This is why, in near-darkness, the world seems drained of colour. You are using your rods.
Human beings have three types of cone cells, each one tuned to a different part of the spectrum. One type responds most strongly to long wavelengths — the red end — a second type to medium wavelengths in the green range, and a third to short wavelengths at the blue-violet end. Your brain compares the signals from all three cone types together and calculates a colour from the combination. An object that stimulates your red and green cones equally, for example, will be perceived as yellow — even though there is no 'yellow cone' inside your eye.
This three-part system has a name: trichromacy. The word comes from the Greek for 'three colours'. It explains why mixing just three primary colours of light — red, green and blue — can produce almost any colour you can imagine. Screen displays, whether on a phone, a computer or a television, use exactly this principle. Look very closely at a white area of a screen and you may be able to make out tiny clusters of red, green and blue dots glowing side by side.
Not everyone experiences colour in the same way. Approximately one in twelve men and one in two hundred women have some form of colour vision deficiency — often called colour blindness, though total inability to see any colour is actually very rare. The most common type makes it difficult to distinguish between red and green, because the red-sensing or green-sensing cones do not function as expected. The condition is inherited: it is passed down through families in the genes.
Interestingly, some animals have a far richer colour experience than we do. Whilst humans have three types of cone, most birds have four, allowing them to see ultraviolet light that is completely invisible to us. The mantis shrimp, a small marine creature, holds the record with sixteen types of photoreceptor. Whether this gives the mantis shrimp a far more vivid experience of colour than a human being, or whether its brain processes colour in an entirely different way, is a question that scientists are still working to answer.
Returning to that ghostly green image on the wall: it has a name too. It is called an afterimage. When you stare at a red shape, your red-sensitive cones gradually become less responsive — scientists say they become 'fatigued'. When you then look at a white wall, those tired red cones hardly react, but the green and blue cones fire normally. Your brain, receiving no red signal and plenty of green and blue, interprets the combination as cyan — a blue-green colour — which is why the afterimage appears the colour it does.
Colour, it turns out, is not really a property of objects at all. It is something that your brain constructs from incomplete information, working at extraordinary speed to produce the vivid, detailed picture of the world that you see in every waking moment. The red of an apple, the blue of the sky, the green of a summer field — none of these colours exist 'out there'. They exist only inside your head.