Colourful Sky in Leonardo’s Eye

There are very few people in human history who have combined arts and science like Leonardo da Vinici. He was a polymath: a great painter, inventor, sculptor, scientist and as you will see – a keen observer of nature, and many more. One of the great aspects of Leonardo is that he recorded his observation as texts, which gives us a deep and direct insight into his thinking.

All of us have been captured by the beauty of sun lit sky. It has made us gaze and wonder about its colour. Of course, it has inspired a countless number of artist and scientist to ask the question : what is the origin of colours of a sun-lit sky ? Leonardo himself was fascinated by this question, and led him to view this question both as a painter and as a scientist. Thanks to the great work of J.P. Richter, who has translated the Literary works of Lenardo da Vinici into english (available free online), we obtain a direct peek into the mind of Leonardo which is an everlasting treasure trove: more you dig more you get.

The title page of the translated book

In the collected works, what has caught the attention of scientists is a chapter titled “Aerial Perspective”. In there, Leonardo is trying to converse with his fellow painters on how to create perspective in paintings.  While doing so, he makes some vital observation and proposes hypotheses, and further discusses about some experiments to test them. Leonardo was a keen observer. His approach to art was heavily influenced by an analytical way of looking at the problem at hand. In his writings, he appeals to painters to pay close attention to angles and perspectives in the geometry. In order to attain precision he gives elaborate explanation based on his observation. Below is an example where he explains how to represent the atmosphere in paintings.

“Why the atmosphere must be represented as paler towards the lower portion? Because the atmosphere is dense near the earth, and the higher it is the rarer it becomes. When the sun is in the East if you look towards the West and a little way to the South and North, you will see that this dense atmosphere receives more light from the sun than the rarer; because the rays meet with greater resistance.”

It is remarkable how his efforts to create a painting inspired him to go deeper and hypothesize a physical phenomenon. Below sentences reveals a connection he makes between colour of the sky and the presence of “insensible atoms”.

“ I say that the blueness we see in the atmosphere is not intrinsic colour, but is caused by warm vapour evaporated in minute and insensible atoms on which the solar rays fall, rendering them luminous against the infinite darkness of the fiery sphere which lies beyond and includes it”

Although, now we know that light scattering from molecules (not atoms) as the reason for colourful sky, we need to really appreciate Leonardo’s quantum leap of thought. Remember, his texts are dated around late 1400s or early 1500 AD, where the presence of atoms and molecules were not yet verified. As a person with scientific aptitude, Leonardo not only hypothesized, but also tested them with experiments. Below he refers to a beautiful experiment with smoke and the perception of colour arising due to the background.

“Again as an illustration of the colour of the atmosphere I will mention the smoke of old and dry wood, which, as it comes out of chimney, appears to turn very blue, when seen between the eye and the dark distance. But as it rises, and comes between the eye and the bright atmosphere, it at once shows of an ashy grey colour; and this happens because it no longer has darkness beyond it, but this bright and luminous space.”

To me this is nothing but a first rate example of looking at nature through a scientific eye, and adapting this view as means to a certain end. It is a tribute to Leonardo who paid such meticulous attention to details, and attempted to explain an unexplained physical phenomenon – all in the name of getting a painting right !  This is also a wonderful example of how aesthetics and science combined in the mind of Leonardo, which further led to some breathtaking work. After all, science and arts are two aspects of human expression, and Leonardo combined them effectively.

There is another lesson we can learn from such endeavours: observations play a key role. Sometimes, when a student is working in a laboratory, she or he may wonder why one should keep records of ones observations. Well, to them I say – look at Leonardo, he took an important step to write down his observations and this served not only as a template for further exploration, but also clarified his thoughts about the phenomenon he was interested in. Writing this way serves two purposes: one is to record the observation at the moment of exploration and other is to seed new thoughts and questions that can be derived out of these recordings.  You can surely get a lot out of this approach – give it a try.

Coming back to the sky – what is the exact origin of its colour? It took almost 400 years after Leonardo’s observations for someone to come up with an ‘accurate’ answer. And that person was Lord Rayleigh (actual name: John William Strutt). In his remarkable research paper published in 1899, Rayleigh explained the blue of the sky as due to molecular scattering. The opening paragraph of this paper is historic and reproduced below- 

“This subject has been treated in papers published many years ago. I resume it in order to examine more closely than hitherto the attenuation undergone by the primary light on its passage through a medium containing small particles, as dependent upon the number and size of the particles. Closely connected with this is the interesting question whether the light from the sky can be explained by diffraction from the molecules of air themselves, or whether it is necessary to appeal to suspended particles composed of foreign matter, solid or liquid. It will appear, I think, that even in the absence of foreign particles we should still have a blue sky.”

The final statement is significant as it recognizes that light scattering can occur purely due to molecules in air, even after discounting the contribution of suspended particles. Rayleigh gave his famous formula in 1871, which drew an inverse relationship between the intensity of scattered light from a very small particle (compared to wavelength of light) and the fourth power of the wavelength of light. In other words, smaller the wavelength of light (violet-blue in case of visible light), more will it be scattered from the molecules in the sky, and hence the blue colour.

One may wonder why not a violet coloured sky. After all, if the inverse relationship between scattering intensity and wavelength holds good, then according to visible colour distribution (VIBGYOR), violet should be the dominant colour of the sky. The answer to this puzzle is a complex one. Mainly because what we perceive as blue is due to a combination of at least three concomitant effects: Rayleigh scattering, human perception and the background in which the scattered light from molecules are observed. Although the colour of the sky is beautiful to perceive from earth, the intricate understanding of the optical processes in atmosphere of planets, including earth, is still a work in progress.

The foundations laid by Leonardo opened a new line of thought, and Rayleigh put forth an important explanation that forms the basis for a majority of studies on light scattering since 1900s. We will revisit Rayleigh and his work many times in this blog; meanwhile enjoy watching the sky with your scientific eye – after all sky is no limit for science!