Colour perception

This is a red tomato. Isn't it? To your eyes it appears red, and I claim that it is red. But how can you know that your red is the same as my red? Well, you can't because colors don't really exist at all.

At least not anywhere outside of our heads. Come along and you'll see for yourself. Here comes a bunch of light rays from the sun. We're going to take a closer look at those. ​A beam of light can be described as a wave motion where the actual waves are teeny-tiny, only a few hundred billionths of a meter. You sometimes hear that white light consist of all the colors of the rainbow.

But now we're going to be a bit picky. White sunlight consist of beams of light of different wavelengths, and we humans perceive light of different wave lengths as different colors. We're going to sort these light beams according to their wave length to make this clearer. ​ In order to sort light according to wave length, we use a prism - a triangular piece of glass. We send this bunch of mixed light beams through the prism. When the light goes from air to glass, it is refracted and changes direction.

Those beams of light that has the greatest wave length, the lowest frequency, are refracted the least. While beams of light with shorter wave lengths are refracted more. The effect is that when light exits on the other side, it is dispersed and sorted according to wave length. Long wave lengths at the top and short at the bottom. We have got a light spectrum.

In a moment, we will let the light waves in the spectrum meet an eye. But, first, we'll look closer at how human eye functions. At the back of the eye is the retina, and behind it your cells sensitive to light. Some of those are especially sensitive to the light of certain wave lengths. Those cells are called cones, and there are three different kinds.

One type of cones react mostly on light with long wave lengths. Others see the medium wave lengths best, and the rest of the cones react strongest to the shorter wave lengths. Let's give it a try. Bring on some light with long wave lengths to the eye. The long sensitive cones wakes up, and sends electrical signals to the brain and this is when it happens.

We see red. The brain interprets the signals from the long wave sensitive cones as red light. Let's change and send light of medium wave length now. The cones that are most sensitive to these wave lengths reacts and sends signals to the brain which sees green. Can you guess now what happens if we send light waves of the shortest wave lengths to the eye?

Yes, blue. The human eye has receptors which are tuned to three different colors - red, green and blue. When light of different wave lengths hit the cones as the same time, our brains combine them, so that we can perceive other colors than red, green and blue. Colors are in the eye of the beholder and brain. Colors are in the eyes and brain of the beholder.

That is why you cannot know whether my red is the same as your red.