For our GeoPicture of the Week, we’ve gotten used to pictures of minerals, fossils or geological phenomena – but today, I want to show you something different: a stunning, complete, double rainbow:
The picture was taken from a helicopter flying over Cottesloe Beach in Western Australia. Rainbows appear because water droplets act like prisms, separating visible light into its constituent spectral colors – the colors of the rainbow. When the bands of color reach the far end of the drop, they bounce back toward the sun; people with their backs turned on the sun can see the rainbow – or more likely, half of it.
All rainbows are round, but we almost never see it. Seeing a full circle requires a viewing area with plenty of droplets in all directions, like for example when you are flying, like in this picture.
The second, dimmer rainbow appears when light bounces two times in the water droplets before coming back. For this reason, it is much dimmer than the first one.
I would consider that to be an optical phenomenon called a glory. The sun is low hence the background. Most cases of glory are seen when the sun is high and the center is the observer’s shadow on clouds below. Double rainbows are not rare.