Eva Crossan, of Balclutha, asks :-

While flying a helicopter my Dad saw a rainbow which was a complete circle. Are all rainbows complete circles?

John Campbell, a physicist with an interest in light and colour in the atmosphere, responded.

A rainbow is the coloured bow we see whenever raindrops are illuminated by the sun behind us. They can be full circles if the conditions are right.

When a ray of white light from the sun enters a spherical droplet of water it travels through the droplet, reflects off the rear surface and re-emerges from the front surface. If we look away from the sun we see a circular bright bow that appears to be at the same angle across as that formed by about four hand spans at arm's length. (42 degrees).

The colour display of the bow comes about because in water blue light travels slightly slower than red light so a blue ray is bent through a larger angle than the red ray. Hence the outside of the bow appears iridescent red with the other colours predominating in turn through to violet on the inside of the bow.

We normally see only part of the circular arc of the rainbow. The lower the sun the more of the arc we can see. If we stood on a sharp mountain or an isolated high building or in a helicopter, with the sun low and behind us and raindrops in front of us, we could see most of the arc.

You can produce your own rainbow by having your back to the sun and spraying a hose into the air. It shows more clearly if there is a dark background, eg dark clouds or dark hills. If the sun is low in the sky (ie near dawn or dusk) and you climb a ladder, you might then observe the full circle of the rainbow, centered on that part of your shadow where your eye is. Hence there are no ends of rainbows for pots of gold to hide.

But was it a rainbow that your Dad saw? If he was looking down into clouds or mist it will almost certainly be due to another effect, back scattering. That colour display is very much smaller than a rainbow. It could be covered by not much more than a thumbnail at arm's length. If he was looking up into high altitude whispy clouds then it would have been a Sun-Dog which is cause by refraction in ice crystals making up the cloud. Let me know which it was.