Higher Order Rainbows - OPOD

Last updated on September 16, 2023

Higher Order Rainbows: A Spectacular Phenomenon

Rainbows are a mesmerizing natural phenomenon that never cease to captivate our imagination. We are all familiar with the primary and secondary rainbows, which result from one and two internal reflections of sunlight inside raindrops. However, these rainbows are just the beginning. There are higher order rainbows that continue to form as light bounces multiple times within the raindrops. These higher order rainbows become progressively fainter and more dispersed, making them incredibly challenging to observe in nature.

The Elusive Nature of Higher Order Rainbows

Higher order rainbows are a sight to behold, but their rarity and inherent dimness make them elusive to the casual observer. These rainbows result from multiple internal reflections within raindrops, causing the remaining reflected light to weaken with each reflection. As a result, higher order bows become increasingly faint and their colors spread over larger areas of the sky. It's no wonder that these magnificent rainbows are rarely seen in nature.

Laboratory Reproduction of Higher Order Rainbows

While natural high order rainbows pose a significant challenge to detect, researchers have successfully reproduced them under controlled laboratory conditions. By illuminating a single water drop or water column with white or laser light, scientists can precisely compute and observe higher order bows. This controlled environment allows for the study and documentation of these elusive rainbows in detail.

The Quest for Natural High Order Rainbows

The search for natural high order rainbows has been an ongoing endeavor, with each increasing order presenting greater difficulties in detection. However, advancements in detection techniques offer hope for uncovering higher order rainbows in the sky. Continuous video monitoring, combined with the use of polarizing and narrow band filters, meticulous shielding of optics, and minimizing stray light, can enhance the chances of capturing these elusive phenomena. Additionally, sun tracking mounts, clean air, dark sky backgrounds, and intensive image processing techniques can contribute to the successful detection of higher order rainbows.

Surprising Discoveries: Third and Fourth Order Rainbows

While the detection of higher order rainbows remains a challenge, some surprising discoveries have been made. Ordinary cameras have managed to capture third and fourth order rainbows in single images, suggesting that these rainbows may be more accessible than previously thought. In fact, a garden lawn sprinkler can create the perfect conditions for capturing these elusive rainbows, serving as a "halfway house" between laboratory conditions and natural settings.

The Fascinating Spectrum of Higher Order Rainbows

Higher order rainbows exhibit unique characteristics and can be identified based on their order and appearance. Here is a breakdown of the different orders of rainbows and their distinct features:

• Zero Order Glow: This glow results from rays passing through raindrops without reflection. It adds to the difficulty of detecting higher order bows, particularly when they are positioned sunwards.
• First and Second Orders: These are the primary and secondary rainbows that we are familiar with. They are bright, intense in color, and have narrow widths, making them unmissable.
• Third and Fourth Orders: Positioned sunwards and inside the zero order glow, these rainbows are broader and fainter compared to the primary and secondary rainbows.
• Fifth Order: Nestling partly in the abyss of Alexander's dark band, the fifth order rainbow has likely been captured in photographs numerous times without being recognized.
• Sixth Order: This rainbow sits within the white light disk of the primary rainbow, making it difficult to detect.
• Seventh Order: Positioned 60° from the sun and clear of interfering light, the seventh order rainbow offers more hope for detection. However, it is broad and very dim, presenting a significant challenge.
• Eighth Order: Deep within the sun's glow, the eighth order rainbow remains elusive for most observers.
• Ninth and Eleventh Orders: These rainbows can be observed in clear skies, but their detection requires advanced equipment and precise conditions.

The Future of High Order Rainbow Observation

As technology and detection techniques continue to advance, the possibilities of observing even higher order rainbows become more promising. Continuous improvement in video monitoring, polarizing filters, and image processing algorithms will likely unveil higher order rainbows beyond the current known limits. With a combination of sophisticated equipment, optimal atmospheric conditions, and meticulous observation methods, researchers and enthusiasts may one day capture the elusive ninth, eleventh, and possibly even higher order rainbows.

In conclusion, higher order rainbows are a captivating natural phenomenon that extends beyond the primary and secondary rainbows we commonly observe. While these rainbows are challenging to detect in nature, advancements in laboratory reproduction and detection techniques offer hope for unraveling their mysteries. The ongoing quest to capture higher order rainbows continues, with surprises and breakthroughs awaiting those who are persistent in their pursuit.

Here be Dragons. Not quite. We have not seen all these bows in the sky but we can predict their guise and where to search. The sun is at far right. Angles from it increase leftwards. The antisolar point is at far left. Each rainbow order corresponding to more and more internal reflections is shown in profile. Imagine each one as a circle across the sky sphere. Computed using a Debye series and Philip Laven's MiePlot. Higher order rainbows get very dim and broad. They are artificially brightened here to make them all visible on the same graphic which therefore DOES NOT show relative intensities.

These we have seen.

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’.

Capturing them?

At top is the 'Zero order glow' from rays passing through drops without reflection. The glow increases the difficulty of detecting higher order bows sunwards.

Orders 1 and 2 are the familiar bows known since Noah. Bright. Intense colours. Narrow. Unmissable.

Then sunwards and inside the zero order glow the 3rd and 4th orders appear. Broader and fainter.

The 5th order nestles partly in the abyss of Alexanders dark band. It has probably been photographed many times and gone unnoticed .

The 6th order sits in the white light disk of the primary. Difficult.

7th order offers more hope. 60° from the sun and clear of interfering light. But broad and very dim. The next trophy for someone's wall?

The 8th is deep in the sun's glow.

For the extremely ambitious, expensively equipped with a fantasy lab of rainbow seeking tools, the 9th and 11th sit in clear sky. But see left hand text!

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Reference Atmospheric Optics

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