The Science Behind a Double Rainbow

Looking skywards and laying your eyes on a kaleidoscope of colours is exciting enough. Throw TWO rainbows into the mix and it’s a sight to behold. The rare spectacle isn’t seen often, but when it does, there’s definitely reason to stop and stare. So what causes a double rainbow?

Dual refractions are twice as nice

Science reveals that the double effect is triggered when sunlight refracts twice within water droplets that are suspended in the air. These raindrops act as miniature prisms that bend the different colours in white light, causing it to shine out in a band of colours. In contrast, single rainbows are formed when light bounces just one of the same droplets. When cast into the sky, the second rainbow appears 10° outside of the primary rainbow. It’s radiated at an angle of 50–53°. The other noticeable difference is the inversion of the colours. Unlike the primary bow, secondary arcs will be red on the inside, and blue on the outside.  

Generally, scientists assert that larger water droplets that have been flattened by the surrounding air are more likely to cause double rainbows than their smaller counterparts. This is because these ‘burgeroid’ droplets have a larger surface area, which supports the reflection of light multiple times.

Sometimes, the droplets can be so large that sunlight is able to bounce three or four times. This forms tertiary or quaternary, however the third and fourth instalments are usually too pale for the naked eye to notice.

More than just a pretty spectacle?

While the Western world puts double rainbows down to science, eastern cultures associate profound meaning with a duo of arcs. To some Feng Shui believers, the phenomenon symbolises transformation, with the first arc representing the material world and the second arc symbolising the spiritual realm.

Want to learn more about the incredible power of light? ‘Bright Lights, Big Science The physical phenomenon that has helped to shape human history and open up our world’ delves into the story of light, and its profound importance in creating life as we know it. From bacteria and to plants to human beings and animals, all species depend on light to survive. As well as acting as the foundation of all existence, it’s also a tool for exploration and advancement, and is renowned as a key ingredient that allows scientists, inventors and pioneers to uncover more about the universe around us.

Image via Flickr Creative Commons. Photo credits: Grant Eaton