As a biologist, I am not an expert on rainbows, but I thought I knew how they work.
A rainbow occurs when the sun is shining into rain, and each water droplet acts as a little spherical prism, splitting the sun’s white light into the colours of the rainbow in a process called refraction.
The sunlight is hitting each rain droplet at the same angle, regardless of where it is in the sky. But the refracted light comes out of the droplet at different angles, depending on its colour (wavelength).
So when we look at one particular droplet, we only see one colour – the one that happens to be refracted in our direction. And if we look at another droplet in a different position, our viewing angle is different, so we see a different colour.
This means that when we look at a rainbow, all the bits of the same colour are droplets for which the angle between the sun and us, the observer, is the same. (Of course, it’s really a succession of droplets falling through that location…)
Now, because it’s the angle to the sun that counts, a rainbow doesn’t move when we move; Just like the sun, it stays put in the same direction. Our brain has learnt that this is the case for objects that are very far away – when we move, stationary things at the horizon appear to move much less than things nearby, something we call motion parallax.
So I was always under the impression, as I’m sure most people are, that rainbows are way up there in the sky. Imagine my surprise when I saw this, from the third-floor balcony of my mother’s apartment. The trees accross the street are less than twenty metres away, and the rainbow must be nearer than that!
Of course, now that I thought about it, the location of the rainbow, if there is such a thing, is the location of the rain droplets that happen to be at the appropriate angle to the sun. And in this case, that was right in front of me, in the space between the balcony and those trees.