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I frequently revisit this one particular theme in my daily musings: the realization that a vast majority of the people living in this world know little about it.
These moments come with no condescension; I bear no resentment towards this universal truth. Here we are, a truly unique species, capable of rational thought, conscious understanding, and self-awareness. We have existed for barely a moment in the earth-timeline, and yet we have hurtled forward at blistering speeds in development and knowledge. Our advancements have outpaced us, and our ability to comprehend the complications of our world is limited to narrow foci of expertise that we contribute to the greater whole. Because of this, few of us have the capacity to understand the details regarding our mundane, day-to-day experiences.
I was struck most recently by this thought when a friend asked me to explain digital information storage. I dipped my toe into the lake of information and recoiled...it is really complicated stuff. To understand it, one has to wade through multiple layers of background information that dives in and out of multiple fields of expertise. I won’t lie – I like to be a know-it-all. I enjoy having an answer for every question, and being smacked in the face with the truth that I can’t understand everything always shakes me a little.
Yet there are some things about the world that I have learned, and retained. Most notably, I enjoy having explanations on hand for the simple events that we observe every day around us. Though my initial hope for this segment was to expound on modern technology and scientific news, I have also found satisfaction in helping my readers appreciate the things that have always been, rather than the up-and-coming. Therefore, this week I will attempt to give a clear, undisputed answer to most commonly asked question by children: why is the sky blue?
What is color?
I’ve overheard people having silly philosophical breakthroughs while trying to discuss the nature of perception. “What if what I see as blue is actually green? What if we’re seeing two totally different colors when we look at the same object?” To scientists, this issue is not so deep. Color is electromagnetic radiation [light] that falls within a certain range of wavelengths, from violet/blue (~400 nanometers) to red (~700 nanometers). These are the wavelengths that the human eye can interpret. When we see green light, it is because an object has emitted electromagnetic waves with a wavelength of 495–570 nm towards our eye. The combination of many wavelengths of visible light all in a bundle looks white. People say that black is “the absence of color,” but something like black paint has to reflect light in order for your eye to see it. It is more appropriate to say that black is what your eyes register when you’re somewhere that is devoid of all light.
The things that we see that aren’t light-emitting – such as paint, asphalt, or skin rather than street lights or TVs– are absorbing all colors of light except the one color that we see. For example, grass is absorbing all wavelengths of light except green, which it is reflecting back at our eyes.
Atmosphere’s role – why space and the sky above the moon is black
Keeping in mind that light is either emitted by a source or reflected off something, you might leap to certain conclusions, since you are brilliant and attentive. The only light source in the sky is the sun. If the sky is showing us color, and its not directly emitting light itself, it means that light from the sun must be reflecting off of something. Specifically, that thing is our atmosphere, the protective gaseous layer of nitrogen, oxygen, argon, and other trace gases that are held in by gravity. To reaffirm this, look at pictures of the earth taken from the moon. You’ll notice that there is no blue sky, only black empty space. This is because the moon isn’t large enough to generate enough gravitational force to hold in an atmosphere, and therefore there are no particles above the surface for light to bounce off of. The sun just looks like a big white hot ball in the sky.
Light scattering off particles
We can easily imagining light reflecting off of a surface, like a ball bouncing off a wall. But gases, which are a type matter, are a little bit more difficult. How does light bounce off something that isn’t a solid? For this application, we use the term scattering, which covers all types of deviation from an original path, whether they’re from reflections, refractions (bends) or other types of redirections. Light can either scatter elastically (with negligible energy transferred or lost) or inelastically (measurable energy is lost). Sky colors come from two types of elastic scattering.
Imagine that a light wave comes in and hits the surface of a big particle. At that surface, some of the light will reflect and simply bounce off like it was a solid wall, but some of the light will transmit through the outer boundary, into the particle. Once in there, it will bounce around, reflecting and transmitting at every surface in a very complicated pattern. The math to handle this process, called Mie (like “me”) Scattering, is very complex. The essential conclusion is that most of the light comes out of this interaction projected along the same path that the light was originally taking, and it usually contains all wavelengths equally, so it is white.
But if the particle is much much smaller than the wavelength of the light striking it, we get a phenomenon called Rayleigh Scattering. In this case, we can neglect all those little inner-bounces, treating the particle like a point and the wave like something that’s alternatively pushing up and down. In the presence of an electric field, small particles will polarize, separating their positive and negative ends, making dipoles. Electromagnetic waves have oscillating electric fields, and so they flip the dipole up and down as they pass. An oscillating dipole will re-radiate light at the frequency that it is being driven at. This re-emitted light comes with an intensity that is proportional to the inverse of the incoming wavelength raised to the fourth power [1/wavelength4]
Why the sky is blue, the sun is yellow, and the horizon is white
In space, the emptiness is black and the sun is white. But from earth looking up, that light from the sun has undergone Rayleigh scattering off of millions of atmospheric molecules. The scattering intensity, proportional to 1/wavelength4 as mentioned above, means that 400 nm light will scatter with nine times the intensity of 700 nm light. That means that blue light scatters much more intensely, while red light tends to follow a straighter path.
When you look directly at the sun, the orange tint that you see is due to the straight path that the light takes to your eyes. The blues and the greens and the purples get knocked off in other directions, while the reds and the oranges and the yellows come straight to you. Overhead, however, when we’re not looking directly at the sun, we’re seeing all that scattering intensity bounced off the atmospheric particles above us and down towards the ground. The blue is a weighted-average of all those highly-scattered colors. In reality, the color is much more violet, but our eyes aren’t as good at picking up those smaller wavelengths, so we see the blue predominantly.
Now remember what I said earlier about Mie scattering? With larger particles, the light tends to scatter forward, as if going around the particle, and it emerges as white light. There are, of course, larger particles in the air as well, and Mie scattering is occurring in the air as well. We don’t really see it overhead for the same reason that we don’t see much red light. Mie scattering tends to travel over our heads unnoticed. On the horizon, however, Mie scattering dominates. That’s why we see the blue color fade to white as we look down towards the ground.
Why the sunset is so pretty
Moving on logically, it’s easy to understand why the sunset gives us such a gorgeous mixture of deep red hues. After traveling in an arc overhead, the sun settles onto the horizon, and once again we are looking directly towards it on a straight path. All those blues and greens are scattered away, and we’re left with the suns reddest rays being transmitted to us and reflecting off the cloud surfaces.
Milk, cigarettes, and clouds – examples of scattering
We see these two scattering phenomena in other aspects of everyday life as well. Clouds, which are made up of large water droplets, are subject to Mie scattering, and therefore appear white. The next time you watch a smoker, notice that the smoke coming directly off the end of a cigarette has a blue tint as it burns, whereas smoke coming out of the mouth after passing through the lungs is white, having picked up water molecules. Skim milk, which contains almost no fat molecules (which are very large) has a very faint blue color when held up to the light, whereas whole milk with its high fat content is white.
The smog myth – is pollution good or bad for your sunsets?
Many people say say that pollution, or the particulate matter that makes up “smog” (either natural or man-made) makes sunsets more vivid. In some cases, this is true. Uniformly sized particles in the air, like ash from a volcano, will scatter more of those blues and greens, leaving an incredibly stunningly red sunset. Man-made pollutants, however, are combination of different-sized particles, many of which are quite large. In that case, Rayleigh scattering is no longer the predominant effect in the sky. The light that is scattered is indiscriminant, and we’re left with a sky devoid of color, or a hazy mix of browns, grays, and whites. When you’re seeing a brilliant red sunset in a place like LA, it’s probably because the smog levels are low that day. This is a common misconception that even I have been guilty of perpetuating in the past. Let it be corrected for you, my readers.
I’ve heard any number of incorrect and often creative answers to the sky-color question, but to be fair, it wasn’t something that I learned until my senior year of college. I also learned way more of the details than I would ever care to share with the casual non-scientist, and I might have stayed up way too late on way too many nights putting together last-minute assignments to have made it very enjoyable to learn at the time. Nevertheless, I think it’s one of those things that more people could stand to know. It happens every day, all around us. Maybe next time I’ll graduate from something easy like the color of the sky into something intimidating like digital information storage.
...or maybe I’ll just keep in simple.
Sorry I missed last week. I hope no one felt too let down. If you did, well...email me about it. And throw in some topic suggestions while you’re at it. It makes my job easier! firstname.lastname@example.org. Until next week!