OPOD - Bows and UnBow

Last updated on September 16, 2023

OPOD - Bows and UnBow: A Detailed Exploration

When it comes to atmospheric optics, the phenomena of rainbows has captivated people for centuries. The play of light and water droplets in the sky creates a stunning display of colors. But did you know that there is more to rainbows than meets the eye? In this article, we will delve deeper into the topic of bows and unbow, shedding light on the science behind these captivating optical effects.

The Creation of Rainbows

Rainbows are formed when sunlight enters raindrops, reflects off the opposite side, and exits the drop at an angle close to the original sunlight direction. This process, known as refraction, separates the colors of light, resulting in the beautiful arc of colors we see in the sky. The primary rainbow, which appears as a semicircle with red on the outer edge and violet on the inner edge, is the most commonly observed type of rainbow.

The Role of Scattering

The colors we perceive in a rainbow are influenced by scattering, which causes the sunlight to lose its greens and blues. When the sun is setting, the light passes through a greater distance of Earth's atmosphere, where scattering is more prominent. As a result, the red end of the spectrum becomes more dominant, giving rise to a red-colored primary rainbow. On the other hand, when the sun is higher in the sky, we see rainbows with a wider range of colors.

The Importance of Minimum Deviation

For a rainbow to be visible, the exit rays from the raindrops must cluster at a specific angle called the minimum deviation or rainbow angle. This angle determines the position of the rainbow in relation to the observer and the sun. It is important to note that not all rays passing through raindrops contribute to the formation of a rainbow. In fact, the majority of rays pass through without any reflection. These non-reflective rays create a different optical effect known as the "zero order glow."

The Enigmatic Zero Order Glow

The zero order glow is a diffuse golden light that occurs when millions of raindrops between the observer and the sun reflect and scatter sunlight without creating a distinct rainbow. Unlike the rainbow, which is characterized by its vibrant colors and clear arc shape, the zero order glow appears as a soft, glowing light that permeates the sky. This phenomenon occurs when there is no angle of minimum deviation for the rays passing through the raindrops. Instead of forming distinct zones of different ray behavior, as in the case of a rainbow, the light is scattered in a more diffuse manner.

The Quest for Optical Surprises

To truly appreciate the wonders of atmospheric optics, it is essential to keep an open mind and observe the sky from different perspectives. While rainbows are most commonly seen opposite the sun, it is worth looking in the opposite direction as well. Often, unexpected optical effects can be observed by being cantankerous and exploring alternative viewpoints. So next time you spot something intriguing in the sky, take a moment to turn around and see if there is a surprise waiting for you.

In Conclusion

Rainbows and their related optical effects, such as the zero order glow, offer us a glimpse into the fascinating interplay between light and water droplets in the atmosphere. Understanding the science behind these phenomena allows us to appreciate the intricate beauty of nature's spectacles. So the next time you witness a rainbow, take a moment to marvel at the wonders of atmospheric optics and the remarkable physics that bring these captivating displays to life.

Bow and Un-Bow

Monika Landy-Gyebnar of Hungary planned carefully in order to capture this panorama of raindrop refractions. At left the low sun has created a highly reddened primary rainbow. At right the sun shines through falling rain to give a wonderful red-golden light - the 'zero order glow' of a rainbow.

"I send you a rainbow & zero order glow panorama I made today .24th April '12. at sunset - just for its beauty.

The zero order glow was terribly vivid. The whole sky had a red/rosy tint but at the western horizon it looked like an A-bomb's light... Many of us had a rainbow today in Hungary and I received 9.5mm rain (most of it fell on my clothes and my hair). The beauty of this rainbow and the glow was in its timing. As a cold front's clouds travelled eastwards across our country I saw their end in the west two hours before sunset. I looked at the velocity of the clouds and said to myself: 'I have to prepare as I'm going to get that cloud gap just at sunset.' And 20 minutes before sunset the cloud gap appeared, the sun shone and made the rainbow and the glow. It was still raining but not too much.. ..so it was simply perfect."

Image ©Monika Landy-Gyebnar

The primary rainbow results from rays that enter raindrops, reflect from the opposite side and leave again deflected almost in the direction of the original sunlight. The two refractions separate the colours. We see a red bow when the sun is setting because scattering has denuded its light of greens and blues.

We only see a rainbow because the exit rays cluster at a minimum deviation or rainbow angle. Put another way, the exit rays form two zones (crossing and non-crossing) whose boundary is a bright caustic sheet.

Textbooks oft forget the rays passing through the drop without a reflection - the majority! With these there is no angle of minimum deviation. There are no two zones of different ray behaviour separated by a sharp and bright caustic. There is no rainbow. Instead there is a lovely diffuse golden glow where millions of raindrops between the eye and sun echo its light - a "zero order glow".

Look for rainbows opposite the sun - then look sunwards. Whenever the sky shows some optical effect be cantankerous and look in the opposite direction, often there is a surprise.

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Reference Atmospheric Optics

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