Why the Sky Is Blue and the Sunset Red

Colour is, in the end, a difference in the size of a ripple. Blue is a fine, tight ripple; red is a long, unhurried one. Read light as a wave and all of this unlocks at once.

The air is full of very small particles. Blue, being tight, catches on them and is flung off in every direction. Red is long enough to step over them.

By day the Sun is high and the light crosses only a thin layer of air, so the flung-off blue reaches your eye from every part of the sky. The sky is not blue. Blue is simply what got bounced our way.

By evening the Sun sits low and the light must cross far more air. The blue is scattered away en route and only the long ripples survive the trip. The red of a sunset is the colour that lasted.

All of it grows from one root: that light is a wave of electricity and magnetism pushing each other along, and that colour is the size of that wave. That was settled in the nineteenth century.

So where is the colour?

To gather it up: the colour is not in the apple, which merely discarded red. Nor is it in the light, which only has ripple sizes.

The feeling of red begins when that ripple lands in an eye, becomes a signal, and is read. Colour exists only where the apple, the light and the eye meet.

1. 1021Al-Haytham — light enters the eye rather than leaving it
2. 1690Huygens — treat light as a wave and reflection falls into place
3. 1801Young — light really does overlap the way waves do
4. 1865Maxwell — light is a wave of electricity and magnetism

The question that remainsIf colour is neither in the object nor in the light but only where the three meet, what colour is a sunset no one watches?