This double rainbow has been photographed in the Ore Mountains in the Czech Republic after a heavy rain. Large part of the sky was still very dark, but the evening Sun started shining brightly right towards the clouds – these are the perfect conditions for double rainbows to occur.
So, what is a double rainbow? A rainbow – any rainbow - occurs when light hits raindrops. As the light enters a raindrop, it changes direction. At the same time, it starts to disperse into a very wide spectrum of colors. And finally, it gets reflected by the back of the water droplet. Part of that reflected multicolored light returns to the observer. And just like that, huge number of raindrops form the multicolored circular arc that is called rainbow.
In most cases, only a single bow of light is visible. However, when the conditions are right, the second bow becomes visible – it is fainter than the primary bow. This is called a double rainbow. It is created by the light getting reflected twice inside the rain droplets that are away from the ones creating the primary bow. Because of that, it is mirrored – while primary bow has blue end inside, the secondary bow has blue colors outside. Technically, all rainbows are double, but usually the second bow is too faint to be observed.
Due to light changing direction as it flows through droplets, the rainbow makes the inner part of the sky brighter. In case of double rainbows, the mirrored second bow illuminates the outer part of the sky, leaving the sky between the two bows unlit. This area of the sky is called Alexander's band, named after Alexander of Aphrodisias who first described the phenomenon around 200 AD.
Rainbows of order higher than 2 are exceptionally rare, but they do occur in the nature and are very faint and hard to spot.