Our two familiar rainbows come from one and two internal reflections of sun rays inside raindrops.
Rainbows do not stop there. There are more. Light continues to bounce inside the drops forming higher and higher order rainbows. At each reflection some light leaves the drop to form a bow and some is internally reflected. The remaining reflected light is correspondingly weakened. Higher order bows get progressively fainter.
There is another effect. Rays forming each rainbow’s rim – the minimum deviation or caustic rays – get closer and closer to the drop’s edge. They leave the drop at increasingly oblique angles and the colours are dispersed wider and wider.
Higher order bows get intrinsically weaker and their colours are spread over greater areas of sky. No wonder that they are not seen casually.
High order bows are now easily produced under laboratory conditions with white or laser light illumination of a single water drop or water column. We can compute them exactly.
Natural high orders in the sky are quite another matter. The long sought 3rd order was first imaged by Michael Grossman in May 2011 and the 4th orders a month later by Michael Theusner. The 5th order, partly visible in Alexander’s dark band, succumbed in 2012. With each increasing order the search difficulties increase.
Yet detection techniques improve all the time. Orders beyond five will surely yield to use of continuous video monitoring, switching polarizing and narrow band filters, scrupulous shielding of optics and minimising stray light, sun tracking mounts, combined with clean air, dark sky backgrounds and intensive image processing. Away from conventional cameras?
This might not be needed! Some 3rd and 4th order rainbows have been captured in single images from ordinary cameras. Go for it! And as a ‘half way house’, a garden lawn sprinkler could give just the right conditions to get them.
The next few OPODs will feature higher order bows in a ‘High Order Rainbow Festival’. |
