Raindrops are round balls of water. When sunlight hits the ball, it goes inside and some of it bounces off the inside of the wall twice and is then reflected back to you.

The rest of it just carries on through.

The angle between the original ray and the one reflected back to you is between 40 and 42 degrees. Why the spread? Because the white light from the sun is actually made up of colours with different wavelengths and when they bounce around inside the raindrop, they bounce at slightly different angles depending upon their wavelength. These different angles separate the colours. into what we think of as the seven colours of the rainbow – red, orange, yellow, green, blue, indigo, violet. But they’re not actually separated as there’s overlap between each pair of colours.

In order for you to see these colours they’ve got to be coming straight from the water droplets to your eye and that can only be the case if, from the raindrop’s position, the line to your eye is 40 to 42 degrees from the line to the sun.

Because the sun is so very far away, its rays are as close to being parallel as makes no difference. So the raindrops that “qualify” are in a circle with you at the centre.

You only see that part of the circle that is above ground because there are actually no raindrops (or sunlight) below ground!

Can you see that the higher the sun is in the sky, the less of the full circle you will see?

But if ever you’ve seen a rainbow from an airplane window it would have been a full circle.