Walking trough the park today we were caught by a heavy rain shower that literally came out of the clear blue sky. Whilst dashing through town to our car we spotted a spectacular rainbow. In fact it was a double rainbow, which many people will be familiar with. The lower (primary) bow was interesting though, as it appeared to show fringes of green and pink inside the violet edge.
When I got home I learned that we had seen a Supernumerary Rainbow. Luckily I caught a nice photo of it (the best camera is the one you have on you) which is shown above. I also enhanced the image to show the fringes better, this is shown below.
Your common-or-garden rainbow is created when light from the Sun is reflected around inside raindrops and projected back at you (see image below). You stand with the Sun behind you and the rain ahead of you (or all around you) and if the conditions are right you see the rainbow forming an arc. The arc forms at an angle of approximately 42 degrees from the axis of the Sun’s rays. The rainbow is not a physical entity but an optical effect and so it moves as you do – meaning you cannot find the end of it, let alone a pot of gold.
Supernumerary bows are created because different rays of light are actually deflected within each raindrop slightly differently. Imagining a raindrop that is roughly spherical, as in the image above, rays that are incident near the top are deflected out at slightly different angle than one incident near the middle. The rays have fractionally different path lengths.
When light rays of different path lengths exit raindrops, they interfere and either enhance or cancel out particular colours. In reality there are many rays entering many raindrops over a range of incident angles. Over the whole range of angles the wave pairs alternately reinforce or cancel each other to produce light and dark fringes. Each bright fringe is a supernumerary bow.
If the raindrops in the air are all about the same size then this means this effect becomes pronounced and you can easily see the supernumerary rainbow. Rain showers with lots of different raindrop sizes don’t produce supernumerary bows.
In reading up on this topic I also read about lots of other types of rainbow I’d never thought about, including red rainbows that occur at sunrise or sunset and so lack the blue chart of their spectrum, reflected rainbows and zero order rainbows which aren’t rainbows at all but enhancements of the Sun’s apparent brightness due to a rainbow occurring.