Why is the Sky Blue?

My mother insists that Google is the greatest invention that has happened in my lifetime. Not because it allows effortless navigation of the internet, but because it catered to my insatiable curiosity I had as a kid. She often reminds me that my second word was ‘Why?’. It was certainly my most used. I remember walking along the cliffs of Guernsey around age 8, likely with mud covering my lower half, asking my mum ‘Why is the sky blue?’, ‘Why does the sea go in and out?’ and other questions it is impossible to explain to an eight-year-old. Luckily my mum could say ‘Google it.’ I would dutifully run to the one computer in the house: a large cream box underneath the stairs with a dial-up cable plugged into it. I would shut down the website about Pokémon emerald I was creating and type into google ‘Why is the sky blue?’. About twenty seconds later, once the all text page had loaded, my mind would be overridden by science and equations more than a decade beyond my eight-year-old minds capacity, at which point I’d get distracted by something and my mind would move on.

“Do not grow old, no matter how long you live. Never cease to stand like curious children before the great mystery into which we were born.” — Einstein

Curiosity is one of the most engaging yet playful emotions there is. Its addictive and truly rewarding. Yet in adulthood it is often lost. Studying at university for long hours, predetermined by a schedule of work often drains curiosity for a subject. There have been long periods where the love of science and learning has faded, and my university life has been a monotonous, exhausting trudge through academic text. Currently however, that is not the case.

I love the subject of Materials Science. Taking each material that the man-made world consists of and breaking it down. Understanding the mechanisms at play that give that material its unique properties. Understanding how we can change the material to change its properties, how it evolves over time and how its changes depend on the conditions it is in. The collective materials science knowledge has allowed us to build structures hundreds of floors tall, have jet engines operating near thousands of degrees Celsius and create machines with greater mathematical power than the entire world in the 1900s that are smaller than the palm of a hand. Everyday miracles.

When you zoom in and question even the most benign aspects of the world, there is a depth of complexity that is fascinating.

I aim, in a series of blogposts to ask simple questions of the world that we live in. The sort of questions that an innocent 8-year-old curious child would ask of their parents. I hope the title will make you curious and that reading the article will fulfil the curiosity and leave you feeling both satisfied and hungry for more knowledge. I am writing the articles such that you do not need a science degree to understand them, yet they will be sufficiently scientific to inspire that addictive curiosity for more knowledge.

The first question is the epitome of innocent child questions. “Why is the sky blue?”. It feels fitting to start this series with that.

Why is the sky blue?

The first person to have explained, with reasonable accuracy, why the sky is blue was Leonardo da Vinci in 16th century Italy. Da Vinci was a man who embodied childhood curiosity throughout his life. In his notebooks it was not uncommon to find entries on his to-do list such as “Describe the tongue of a woodpecker and a crocodile’s jaw.”. Put on there, it seems, out of pure curiosity. The crocodile’s jaw being of note because it has two hinges, not one, allowing for a greater clamping force and the woodpeckers tongue being truly bizarre but allowing it to repeatedly smash its beak into a tree with 10 times the equivalent force that would kill a human doing so.

Leonardo da Vinci (1452–1519)

Leonardo is one of the greatest geniuses to have ever lived, having a life dedicated solely to following his curiosity. He accrued expertise on a vast range of topics and, along with being one of the greatest artists to have ever lived, was the forefather of several scientific disciplines including anatomy, palaeontology, optics, civil engineering and astronomy.

Aligned with this curiosity, around his mid-50s Leonardo questioned why the sky was blue, dutifully recording his investigations in a notebook now owned by Bill Gates.

The overwhelming majority of the light from the sun travels straight through the atmosphere and hits the earths surface. This is the typical white light we associate with sunlight; and this is not blue at all. What is blue is the sky. How does this happen?

Leonardo observed, whilst on a hike, that the sky appeared to get a richer more beautiful blue as he climbed higher. While many of us would simply admire the view on a mountain hike, for Leonardo like everything it was an exercise in observation. Leonardo was aware of moisture in the air from his studies on rivers and the water cycle; where he had correctly concluded (for the first time in history) that all water falling as rain came from evaporating water on the ground. He suggested water vapour consisted of tiny droplets, too small for the eye to see. And that there were far more of these particles at lower altitudes, nearer the puddles and lakes they originated from; than at higher altitudes.

He suggested that the blue tint of the sky came from the scattering of light off these tiny droplets. The sky was darker at higher altitudes as there was less scattering due to less particles. The closer to sea level, the thicker the atmosphere and hence the brighter the blue tint was.

At heights near the top of Everest, the sky appears almost indigo. Less scattering hence the blue is a lower intensity (darker).

To test this scattering idea, Leonardo set up a dark room, with a beam of sunlight going from one side to the other. He set up a wood fire in the path of the beam, that billowed smoke. A beam of white light entered the room; yet Leonardo observed a light blue tint to the light whilst looking at the scattered light perpendicular to the path of the beam.

Smoke has an extremely high density of particles in it. Therefore, it will scatter light far more often than regular air. This means that we can achieve the same amount of scattering in hundreds of metres of air, in just a small volume of smoke. By observing this blue tint, Leonardo effectively confirmed, that it is the small particles in air that scatter light and somehow this leads to a blue tint. He had no clue why the scattering led to a blue tint as a pose to a tint of any other colour.

Why blue?

It wasn’t until hundreds of years later that work by quantum physicists, notably Lorentz, Mie and Rayleigh, would come together to explain exactly what was happening during scattering that created the blue tint.

Electromagnetic wave. Red — Electric field, Blue — Magnetic field.

Sunlight is, at its most fundamental description, a collection of electromagnetic waves. Perpendicular electric and magnetic oscillating sine waves. The wavelength, the distance between peaks on one of the waves, defines its colour. In sunlight there is a huge spectrum of wavelengths present, far broader than the range our eyes can detect. Visible light varies from 400 nanometre blue light to 700 nanometre red light.

In the air, as Leonardo rightly concluded, are lots of tiny particles much like those found in wood smoke. These particles are smaller than the 400nm blue light.

Waves generally only interact with things about the same size as their wavelengths. One can picture the entrance to a harbour, where ripple patterns are only achieved if the spacing between wave peaks is roughly the same as the harbour gate width.

Waves only interact with objects or gaps similar to their wavelength.

Similarly, light will only interact with particles about the same size as its wavelength; the more dissimilar the sizing, the less interaction.

It turns out that the size of water vapour clusters and soot particles in the first few kilometres of the atmosphere are smaller than the wavelength of blue light. Therefore, much smaller than the wavelength of red light and the rest of the spectrum. This means that blue light is scattered by these nanoscopic water droplets more than the light at the longer end of the spectrum. Blue light is more than three times more likely to scatter than red light.

The overwhelming majority of light passes straight through the atmosphere with negligible scattering. This is the white bright sunlight we associate as white sunlight. A small amount however bounces around the sky getting progressively bluer with each successive scattering event. Therefore, as we look up to the sky, our eyes detect these beams of light bounced towards the earth as blue.

Why does the sky turn red at night?

I truly hope you are now asking “What about at sunset? Why does the light turn red?” It’s for the exact same reason that the sky is blue during the day.

During the day, the sun is above us. It comes down vertically through the sky, passing through a few kilometres of particles in the lower atmosphere. Meaning that only a tiny fraction of all blue light is scattered. At night however, the sun is setting over the horizon, hence the distance of atmosphere it passes through increases. This means that more and more of the blue light is scattered. This means by the time the sunlight reaches us, it is void of blue tones leaving it golden. If we are in England at sunset lots of the blue waves will have been scattered off over the Atlantic. The light when it reaches us is a beautiful mix of red and yellows. The east gets the west’s leftovers.

The sky in a material.

So far, we have established that blue light is scattered by particles that are just smaller than the wavelength of blue light. Is there any way we can use this knowledge to create a solid material that looks like a piece of blue sky in our hand?

The answer is yes, you can see a photo of it above. It’s called aerogel and its beautiful. This material is an interconnected network of nanoscopic pores, just hundreds of nanometres across. Most of the light incident on the material will pass straight through it, yet just like particles in the atmosphere, this interconnected network of interfaces scatters some of the blue light. This gives the block a wonderful blue glow as scattered light escapes and travels to our eyes.

The material is often described as “frozen smoke”, a brilliant indication of how ahead of its time Leonardo’s wood smoke experiment truly was.

Aerogel has a porosity of between 90% and 99.9%; it is almost entirely empty space; creating it is therefore difficult. How do we create a structure that is so sparse, yet mechanically stable? We create a gel (an interconnected network of silicon and oxygen atoms) in a solvent. Once the network is formed, we simply have to remove the solvent; leaving the dried network behind.

This is harder than it seems however, as liquid solvent as it escaped the gel would have enough tension to rip the network structure apart, destroying it. We therefore cool the sol-gel to extremely low temperatures and by manipulating pressure drive the solid solvent to sublime, turning straight into a gas. Gases have no surface tension and hence the delicate network is left intact. Once all the solvent has sublimed, the network can be warmed back up to room temperature.

This seems like a huge endeavour to create a block of material, and indeed it is. It is extremely expensive to do. What then are the purposes of this wonder material?

Incredible heat resistance.

In materials science, all properties come from the microstructure of the material. This material has incredibly high porosity. Its up to 99.9% empty space! Its like a nanoscopic polystyrene. Therefore, this material is incredibly insulative; and it is mostly transparent. Its proponents fancy it as one of the insulators of the future, sitting between window panes and keeping the inside of our houses toasty in the winter months.

Stardust collector using aerogel.

However, it has only been used once on any project of scale. It was used in a large array, resembling a tennis racket, to catch stardust. Star dust is incredibly fragile and travels extremely quickly through space. Impact with any other material would have shattered the stardust, but when stardust hit into aerogel, it embedded itself within the material. The stardust catcher could then be retrieved and returned to earth for stardust study. Although incredible, this can hardly be described as an everyday example.

The material is therefore not one of great practical use, but a novel material. It exists due to curiosity; with scientists trying to create the least dense material possible. It scatters light much in the same way as the sky does, a fact that has little to no impact on human life but is fascinating nonetheless. Due to its high cost, it is useless, yet it is one of the most interesting and unique materials humans have ever created.

Aerogel is a solid material that represents curiosity for curiosities sake. It may not impact your life in any way to know why the sky is blue, but that is exactly why you want to know anyway.

If you have enjoyed this read then please give it a clap and share it. I have loved writing it and hope you feel inspired to let your innate curiosity shine.

-Jack Aspinall. Materials Science Undergraduate at the University of Oxford.