HOW RAINBOWS WORK

It's a very pleasant, sunny day. The sky is springtime blue and the air carries a little promise of warmth, hinting at a season yet to come. Not much chance of a rainbow today.

I've been thinking a lot more about rainbows after yesterday. I'm not far off the mark calling them magic, you know. They are, after all, an illusion: a kind of optical trick of the light. As with all illusions, behind the scenes in the structured world of science, there is a perfectly rational explanation with which we're all familiar: sunlight refracts (bends) through water droplets in the air, the light is dispersed into its constituent colours by the raindrops and it forms an arc of coloured wonder in the sky. Magic.

Although, if you dig a little deeper, it doesn't take long to get to a world that's much more magical and tricky to explain. For example, why does light bend in water anyway? What's happening to it as it passes through? Why don't you see a rainbow when you look towards the sun? If light is made out of particles (photons), why does it bend and refract at all? How can you disperse a particle?

Now I don't think this is necessarily the place to discuss phase-velocity or quantum mechanics. Those are huge, imposing words for a light-hearted blog. Five-Year-Old-Me doesn't care anyway, he just loves rainbows.

Rainbows have a lot to do with change. When the waters recede and Noah steps from the Ark, it's a rainbow that teaches him that the world is different. When the storm rumbles off into the distance and the sky is washed with fresh sunlight, that's when you see the transition from darkness to light. Everything has changed. Even the naughty leprechaun, chuckling away from his unsearchable treasure at the end of the rainbow, promises change of golden proportions.

The physics is about the way things change too. Light changes at the moment it meets a raindrop - its direction changes, its velocity changes, it bends and twists into something new and wonderful. The constituent colours (wavelengths) interact with the water molecules in different ways because they're all slightly differently spaced. Reds, oranges, yellows, greens, blues, indigos, violets... all emerge from the wall of water at different angles and form a rainbow. The light itself has transformed at the point very point where heaven intersected earth.

I quite like this idea of change. There's an air of it in lots of areas in my life at the moment and I'm fascinated by my reactions. It seems I swing wildly between excited and furious, humiliated and hopeful, depending on what it is. The one thing I don't want it to be is boring.

But as Five-Year-Old-Me has worked out, the place where sunlight meets rainfall, where art meets science and where storm clouds meet sunshine... well it should be anything but that.