How to: Be Uninteresting About Atmospheric Optics
When I was asked to do a piece for this month’s Marketing campaign, I decided to ask my kids what they thought I should write about. They hated all my ideas, including explaining why we see the colors in the sky that we do. Nevertheless, I went into excruciating detail of how it all worked and that for over a hundred years after the answer was finalized by Lord Rayleigh reliable sources like the Encyclopedia Britannica continued to spread an imperfect understanding of this optical phenomenon. So, when my kids yawned at my reply and said that I was “boring,” I knew that I had my answer. Get your pencils and notepads – let’s get ready to make some friends!
The American Meteorological Society defines Atmospheric Optics as the study of the optical characteristics of the atmosphere or products of atmospheric processes[1]. The field of study is useful because it describes phenomena like rainbows, mirages, halos, and even the color of our sky. More recently, this knowledge is applied with modern technologies, like spectroscopy, which enables people to make accurate measurements of the atmospheres of other worlds. Worlds that our kids may consider going to once they hear the rest of this.
As my intended audience is already well aware of – the first images from the James Webb Space Telescope were released this week. The James Webb Space Telescope was a fancy thing created by a joint partnership between NASA, ESA, and the CSA. Among the awe-inspiring clickbait of stars being born and infrared images of the distant Universe, there was a spectral plot of an exoplanet: WASP-96b. The image demonstrates with almost certainty that there is water, haze, and clouds in its atmosphere.
The “how” and “why” behind the James Webb Space Telescope results can be attributed to a litany of human endeavors in the field of Atmospheric Optics.
So, before we dig into this fascinating field and specifically address why the daytime sky is blue on Earth, I demand that we look back to see where all these brilliant people who figured it out actually learned the fundamental ideas from. As I occasionally say to my team, “We are all working together for a better future!” They do laugh – and, probably, more than they should.
A Brief History of the Sky’s Blueness
Throughout history, people have tried to understand and characterize the origin of the colors which make up our sky. For example, the earliest serious attempt to explain what was going on with the sky’s color was made by savant Al-Kindi around 850 AD. His description of the sky is striking – that the blueness of the sky could be attributed to “a mixture of the night sky with the light of the dust and haze particles in the air illuminated by the sun.”
During the Renaissance, Leonardo da Vinci made insightful observations about the atmosphere from the challenges of aerial perspective in landscape painting. Here is something we found from his notebooks[2]:
You know that in an atmosphere of equal density the remotest objects seen through it, as mountains, in consequence of the great quantity of atmosphere between your eye and them appear blue and almost of the same hue as the atmosphere itself.
I found a relatively bad example of his statement from the National Gallery of Art – because it is free to use. But, as you can see from his painting, the colors in the background (over her shoulder) do indeed follow the observation of atmospheric hues in the distant landscape.
Leonardo continues to be held as the father of atmospheric optics. However, like all of us, he got a lot right and some of it wrong.
The polymath continued to describe the blue which is seen in the atmosphere is caused by heated moisture particles to luminesce when exposed to solar rays. This was an impressive insight, but it wasn’t perfect. In fact, his misconception took the world by storm and persisted through the Age of Enlightenment and deep into the 19th century.
Interestingly, in 1890 someone came up with a solution to the enigma – his name was Ludvig Lorenz (1829-1891). Think of him as the Montreal Canadiens of hockey. The breakthrough required a mathematical solution of scattering of light by particles with dimensions smaller than and comparable to the incident wavelength.
However, Lorenz, like many of us (not on social media accounts) decided not to share his results to physicists further than his immediate language and cultural foundation dictated.
Which brings us to the now famous, at least to us, John William Strutt (1842-1919). Otherwise known as the Third Baron Rayleigh – and how he had developed the famous equation that pulled everything together. I will explain this in a subsequent article, but it will cover how the dielectric permittivity of a particle relative to its surrounding medium, the scattering angle, number of particles, particle volume, wavelength of incident light, and the distance between the particles and the observer finally changed human history. [4]
As to not leave you in a lurch, Rayleigh went on to incorporate Maxwell’s electromagnetic theory of light into his own texts. This had settled the argument once and for all. However, most still can’t even glean to understand what the heck he was talking about. Even my poor kids.
As always, thanks for reading and I’ll catch you all in the next installment.
References
1 – “Atmospheric optics – AMS Glossary”
2 – L. da Vinci, (1508), The Notebooks, E MacCurdy, ed., Konecky & Konecky 2003.
3 – “Ginevra de’ Benci [obverse], c. 1474/1478 – National Gallery of Art”
4 – Lord Rayleigh, On the transmission of Light Through an Atmosphere Containing Small Particles in Suspension, and on the Origin of the Blue of the Sky. J.W. Strutt, (1899)
By Michael Newel
StellarNet Lead Optical and Systems Engineer
A Brief History of the Sky’s Blueness
How to: Be Uninteresting About Atmospheric Optics
