OPOD - Unweaving the rainbow

Last updated on September 16, 2023

OPOD - Unweaving the Rainbow: A Deeper Dive into Atmospheric Optics

Rainbows have always captivated our imagination with their vibrant colors and ethereal beauty. But there is so much more to these natural wonders than meets the eye. In this article, we will take a closer look at the science behind rainbows and delve into the intricate details that make them truly awe-inspiring.

The Spectrum of Colors

When we think of a rainbow, the classic sequence of colors that comes to mind is red, orange, yellow, green, blue, indigo, and violet. However, the reality is that there is an infinite gradation of hues within a rainbow. Each of these colors is actually a mixture of numerous pure spectral colors.

To illustrate this, let's consider a computed rainbow made by large raindrops where diffraction effects can be ignored. By overlapping a few hundred pure colors, we can create a reasonable representation of a rainbow. Although this falls short of infinite variations, it provides a fascinating insight into the complexity of colors within a rainbow.

The Sharp Edges and Blurred Intensity

In an ideal rainbow, each pure color would have a sharp edge at the rim of the bow and gradually fade in brightness towards the center. However, the presence of the sun as a 0.5° diameter disk blurs the intensity profile of the rainbow. This means that the colors blend together more seamlessly than we might imagine.

Mixing of Colors

As we explore different parts of the rainbow, we begin to realize that each section contains a combination of colors with varying wavelengths. For instance, when we focus on the green part of the bow, we find that oranges, yellows, and reds also contribute to the final hue. This mixing of colors adds depth and complexity to the overall appearance of the rainbow.

Keats and the Magic of Natural Philosophy

The poet Keats once expressed his concern that the study of philosophy might "unweave a rainbow" and strip it of its magic and mystery. However, the truth is quite the opposite. Natural philosophy, or the scientific understanding of the world, actually enhances our appreciation of the rainbow's enchantment.

By unraveling the intricate mechanisms behind the formation of rainbows, we gain a deeper understanding of the magic that lies within the natural world. Rather than diminishing its wonder, scientific knowledge fills our imagination and allows it to soar on powerful wings of curiosity and awe.

The Unending Fascination

Rainbows continue to captivate scientists and enthusiasts alike. There is still much to explore and uncover about these mesmerizing optical phenomena. From double rainbows to supernumerary bows, each new discovery adds to our understanding of the complex interplay between light, water droplets, and atmospheric conditions.

As we delve further into the study of atmospheric optics, we uncover the intricacies that give rise to these breathtaking displays of color. By embracing both the scientific and poetic perspectives, we can truly appreciate the beauty and wonder of rainbows in all their glorious manifestations.

Conclusion

Rainbows are not simply a symphony of colors; they are a testament to the fascinating interplay between light and water droplets in our atmosphere. From the infinite gradation of hues to the blending of colors and the blurred intensity profile, every aspect of a rainbow holds a story waiting to be unraveled.

Rather than "unweaving" the rainbow, the exploration of atmospheric optics enriches our understanding and deepens our appreciation for the wonders of the natural world. So, the next time you catch a glimpse of a rainbow arching across the sky, take a moment to marvel at the science behind its splendor and let your imagination soar on the wings of curiosity.

Rainbow Colours

Rainbows rarely come brighter or more colourful than this. S�ndor P�csai captured it at Davod, Hungary. The bow is made by nearby large raindrops. ©S�ndor P�csai, shown with permission

Red - orange - yellow - green - blue - indigo - violet are the traditional rainbow colours. In fact, there is an infinite gradation of hues.

All rainbow hues are mixtures of the other infinity of pure spectral colours.

At left is a computed rainbow made by large drops where diffraction effects might be ignored. A few hundred overlapping pure colours were used - short of infinite but enough to give a reasonable result. Just 13 are shown as examples at left.

In the rainbow each pure colour would have a sharp edge at the bow's rim at right and tail off gradually in brightness towards the bow centre at left. "Would have" because the sun is a 0.5° diameter disk and blurs the intensity profile.

Take the green part of the bow for instance. Oranges, yellows and reds all add their bit to the final hue. Each part of the rainbow has colours of longer wavelength mixed in .

There is much more of rainbows to unweave. Poet Keats is oft quoted ...

"Do not all charms fly

At the mere touch of cold philosophy?

There was an awful rainbow once in heaven:

We know her woof, her texture; she is given

In the dull catalogue of common things.

Philosophy will clip an Angel's wings,

Conquer all mysteries by rule and line,

Empty the haunted air, and gnomed mine--

Unweave a rainbow.."

Oh! If only the poet had learned a little natural philosophy. He would have known that its touch gives to the catalogue of common things magic and mysteries. It fills our imagination and lifts it on powerful beating wings.

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Reference Atmospheric Optics

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