How do we see colours?
What colour is a red jumper?
You hang a deep red jumper in a dark room and shine only green light on it. Will it look red, green or something else? To work it out, we first need to find out what seeing actually means.
Seeing means catching light
You can only see a thing when light travels from it into your eye. A lamp or the Sun shines on everything around, light bounces off them in all directions and a little of it reaches your eye. In the dark, where there’s no light, you see nothing.
White light is a mixture of colours
Light from the Sun or a light bulb is a mixture of all the colours of the rainbow. Colours differ in their wavelength: red has the longest waves, blue and violet the shortest. A prism or raindrops can split white light into colours.
The eye is like a camera
Light enters the eye through the pupil, and the lens uses it to form an image on the back wall of the eye, the retina. The retina is covered with millions of tiny light-sensitive cells. They send the brain a message about how much light has fallen on them.
Three kinds of cones
Three kinds of cells in the retina, called cones, tell colours apart: red ones respond mainly to long waves, green ones to medium and blue ones to short. The brain compares how strongly each kind responded, and from that it works out the colour.
Yellow from two colours
Yellow light stimulates the red and green cones at the same time. But a mixture of red and green light stimulates them in exactly the same way, and the eye can’t tell the difference. That’s why a screen can make most colours out of just three: tiny red, green and blue dots.
Try it yourself
Put a small drop of water on your phone’s screen and open a white page. The drop works as a magnifying glass, and under it you’ll see that white is really lots of tiny red, green and blue dots. On a yellow area only the red and green ones light up.
Why the jumper is red
The jumper doesn’t give off light itself, it only reflects the light that falls on it. The dye in it absorbs green and blue and reflects mainly red, and that reaches your eye. White paper reflects all colours, a black T-shirt hardly any.
A red jumper in green light
The jumper absorbs green light, and the red it could reflect isn’t in green light at all. So almost nothing travels from it to your eye, and a thing that sends us no light looks black. The jumper will look almost black.
Try it with a tomato
In a dark room, open a picture on your phone that is completely green and shine it on a tomato or a red T-shirt. The red goes dark, almost black. The screen is now giving off only green light, and the red light the tomato would reflect isn’t in it.
Colours fade at dusk
At dusk, rods take over, another kind of cell in the retina. They are far more sensitive than cones, but there is only one kind of them, so they can’t tell colours apart. That’s why colours fade in moonlight and everything looks grey.
What is colour blindness
A colour-blind person usually doesn’t see in black and white. Most often their red or green cones work differently or are missing altogether, and the brain then has trouble telling red from green. It affects about one man in twelve.
Do we see the same red?
Colour is put together by the brain, and when two people agree that an apple is red, all we know is that they both tell colours apart in the same way. Your red might look completely different from your friend’s red. And you’ll never find out, because no one can look inside someone else’s head.
So how do we see colours?
We see light that travels from things into the eye, and the things absorb some of its colours. Three kinds of cones pick up what’s left, and from how strongly each one responded the brain puts together a colour. That’s why a red jumper turns black in green light.