Colours & mixing light

A rainbow after the rain, the glowing display of your phone, a bright red jacket in the sunlight: colours are everywhere around you. But where do they actually come from? The answer is hidden in light itself, and it is more surprising than you might think.

White light is full of colours

White light looks colourless, but in truth it contains all the colours of the rainbow. A glass prism can fan them out: a beam of white light becomes a colourful band from red to violet. Classically seven colours are counted in that band: red, orange, yellow, green, blue, indigo, and violet. In reality they blend into each other without any steps. Exactly the same thing happens in a rainbow, except there millions of tiny water droplets split the sunlight.

How your screen mixes colours

Your screen does exactly the opposite of the prism: it builds colours out of light. Every pixel consists of three tiny points of light in red, green, and blue. This is called additive colour mixing, because light is added. Red and green together make yellow, and when all three channels are turned up fully, you see white.

Why does the tomato look red?

A tomato does not glow by itself. It is hit by white light, which contains all the colours. The tomato swallows most of them and reflects mainly the red light. In physics, swallowing light is called absorbing and throwing it back is called reflecting. Only the reflected light reaches your eye, which is why you see it as red. A white sheet of paper reflects almost everything, a black shirt swallows almost everything.