I was debating on whether to write this one or not, seeing as when I surfed through the internet to see just how many people were interested in this topic, a virtually infinite number of videos and articles came up, all addressing this question in varying detail...
The trouble is though, I don't think many of them explain it very holistically.
Why the sky appears blue - "shorter wavelengths of light get scattered the most..... therefore we see blue..."
So why doesn't the sky appear purple then?
Why does the Sun appear yellow at daytime?
But red at sunset?
Why does the sky appear pink sometimes at sunset?!
What in the **** does "scattering" even mean?!
Expansive is about knowing the "why's" and "how's", so I figured it would be a good opportunity to write a comprehensive explanation of everything that you could possible want to know.... and I mean everything!
So let's get straight to it.
An introduction to the Electromagnetic Spectrum
Firstly, let's explain what blue light actually is, and what makes it different from say... purple or red.
What you must remember is something called the electromagnetic spectrum.
Light, or more specifically visible light, is a form of radiation - just like x-rays, radio waves, microwaves etc....
And they're all essentially made from the same thing - energy.
They are packets of energy that oscillate up and down throughout space-time.
How much they oscillate up and down per second is what we call their frequency, and the length of their individual waves is quite simply their wavelength.
Why am I telling you this?
Well, these 2 characteristics define them, and it's what differentiates the different types of radiation from each other.
The only difference between the visible light that we can see (colours), X-rays, gamma rays, microwaves, radio waves etc, is their wavelength and their frequency.
Starting from the shortest wavelength to the longest, the list looks like this:
Gamma , x-ray , ultraviolet , visible light , infrared , microwave , radio wave
And to help visualise that - see this image below...
Our eyes can only detect a small portion of this spectrum - visible light, which itself comprises of another spectrum that separates the different colours from each other.
Again, the different colours only differ from each other by their wavelength and frequency.
However, some organisms in this world are capable of seeing longer, or shorter wavelengths.
For example, eagles can detect infrared waves which, as the theory of evolution goes, allows them to see the heat signature of their prey.
The order of the visible light spectrum from shortest to longest in wavelength goes as follows:
Purple , Blue , Green , Yellow , Orange , Red
Remember this, because it's key to understanding what the main question of this article is about - why is the sky blue (we'll also talk about why the sky can sometimes appear red/pink at sunset).
So why is the sky blue?
Well, imagine its daytime, when the sky is at its most blue.
At this time of day, you are at the closest point to the Sun that you'll be in for that day, which is why it's usually the hottest at around 12-1pm - you're technically closer to the Sun than at say 6pm or 10am, due to the rotation of the Earth.
When it comes to answering this question though, it's not due to the fact that you are closer to the Sun that makes the sky blue, but it is linked to that.
If you can imagine it in your head, at this time of day you are facing directly at the Sun, so any light travelling from the Sun will travel its shortest distance to you for that day, because you're head on and not at an angle.
Because of this, the amount of atmosphere that the light has to travel through isn't that significant, or at least it's not as much as when you're at a sharp angle to the Sun, for example at sunset when you're facing around 90 degrees away from the Sun (which, again, hopefully you can imagine in your head.)
Why the sky appears blue - The short answer
As it turns out, the particles in our atmosphere tend to scatter shorter wavelength radiation like blue, more so than longer wavelength radiation like red or green.
The longer and far more helpful answer though, goes like this:
The longer answer
As I mentioned earlier, all forms of light are electromagnetic waves - whether that be visible light, ultraviolet, gamma etc....
More specifically though, a simplistic definition of an electromagnetic wave from quantum theory (which is the current accepted model, although it's still a theory) would be that its a disturbance of energy in space-time - a spike in energy that oscillates through space like a wave.
As a quick side note - interestingly, QM (Quantum Mechanics) says that everything can in fact be described by some wave function (given by the famous Schrödinger wave equation) - electrons, protons, neutrons etc are all essentially spikes of energy in a quantum field that while are technically waves, exhibit particle-like properties, and interact with matter like particles do, hence why they're often simplistically refined to as particles.
So that's just something to bear in mind when you hear subatomic particles, whether they be fundamental like electrons and quarks or not, being referred to as "particles" - at the time of writing this, quantum theory proposes that they're actually waves, but their interactions with normal matter can be explained using classical newtonian mechanics, like forces, momentum, electromagnetism etc, so it's easier to visualise them as particles.
When these electromagnetic waves strike an electron, they apply some force to it, which in turns gives the electron acceleration, since Isaac Newton's 2nd law of motion tells us that f =ma, or in words, if you apply a force to a particle, you increase its acceleration.
However, an electron is a charged particle (or wave...), and Larmor's power formula tells us that when charged particles accelerate, they give off or emit their own waves of energy - their own light.
Just to be clear here, we're not talking about electrons jumping energy shells and emitting photons of light to be relieved of any "excitation".
This is purely classical mechanics, in that electrons feel a force when they're bombarded with electromagnetic waves and this in turn gives them acceleration in accordance with Newton's laws of motion.
Because they have gained acceleration though, they radiate their own energy in accordance with Larmor's power formula, which would probably make this article way too long if I went into detail of! I do plan on writing an article specifically about moving charges though, so consider signing up to be notified when that drops!
If you're interested in learning about electrons jumping shells and where light even comes from in the first place, ie how the Sun or even a lamp produces light, then check out this article here!
We're almost there now, so stick with me here!
The trouble is though, electrons won't interact with all waves of energy in the same way.
Remember earlier how I said that waves of energy can be described by 2 properties - wavelength and frequency?
Well, waves of different frequencies will interact differently with electrons, and to make this easier to understand, consider the following thought experiment:
Imagine you're pushing a swing seat.
You can't push with any force at any time and expect the same result.
Sometimes, when you push it reduces the distance over which the swing oscillates (when you push against the movement of the swing), and other times you increase the distance over which the swing oscillates (when you push with the motion of the swing).
Similarly, you have to push with an appropriate amount force over an appropriate amount of time to get the greatest effect - everything has to be timed perfectly, although our minds are remarkably intelligent and quickly learn to get the gist of things!
I know that sounds completely unrelated to our situation with electrons and electromagnetic waves, but surprisingly it really isn't!
The movement of the swing can be represented on a graph like a wave, and so can the force that you apply to it.
Electrons move or vibrate at their own frequencies - their natural frequencies, and when waves of energy of their own frequencies collide with electrons, sometimes they act against the motion of the electron and other times with the motion of the electrons, just like in our swing example.
Additionally, they have to have strike the electron with a very particular force over a certain time period for the maximum effect.
In this case though, the "force" and "time period" can be summarised by the frequency of the wave, since we know that the energy of a electromagnetic wave is equal to its frequency multiplied by Planck's constant (e = hf).
In our case, it's the frequency that correlates to blue light that interacts the strongest with the electron, and we call this mechanism of similar frequencies "resonance".
So just to recap here, when white light (which is a combination of all the different colours) from the Sun hits the atmosphere, it exerts a force on the electrons that make up the atoms in our atmosphere.
When these electrons accelerate due to this force, they emit their own photons of light.
Since the amount that the electrons accelerate will ultimately dictate the intensity of the colour that they give off (think of it as the height that the swing reaches), the light that accelerates the electron the most will be the light that comes out on top.
In the case of atoms of oxygen and nitrogen, the electrons naturally vibrate with a frequency that resonates with blue light (more on that shortly), meaning that blue light is emitted by the electrons far more than other colours such as red.
In fact, blue light is emitted more than 10x as much as red light, and since the blue light is scattered across the sky, the sky appears mostly blue to us.
Something that's good to bear in mind though is that technically the sky contains all colours of light, just blue is far more intense than red and our eyes/brains work by filtering light by intensity.
I know I mentioned this earlier, but I feel it's an extremely important point to remember, since it will save a lot of confusion if you understand this properly.
We're not talking about electrons becoming "excited".
If you've read the other article that I mentioned earlier, then you'll know that light comes from electrons absorbing energy, which causes them to jump to higher energy shells and enter an "excited" state.
Their resolution to this is to release a very specific frequency of light that is unique to every orbital of every element, which is why different substances release different colours of light when they're heated up ie supplied with energy.
The colours that oxygen and nitrogen release form this are not mostly blue, and hence this has nothing to do with Rayleigh scattering!
So if you looked up the colours that oxygen and nitrogen emit (formally called the emission spectrum of x element) and were confused at the result, then this is why!
The electrons in this case are just oscillating more due to feeling a force from the electromagnetic waves, and all accelerating charges (like electrons) release energy.
Anyway, with that in mind, let's return to why blue light isn't actually the colour that's emitted the most...
Technically, UV (Ultraviolet) radiation is scattered the most, and purple light is scattered more than blue, but we'll come onto why the sky doesn't seem purple in a minute.
So the electrons of the nitrogen and oxygen atoms emit blue light more than other colours like red, but the truth is that this mechanism of Rayleigh scattering only happens in the upper atmosphere, essentially near the border between the Earth's atmosphere and space.
The reason for this is because the density of particles can affect how well light is scattered, or more specifically the distance between particles matter.
The visible light spectrum covers the range of electromagnetic waves of around 300-700 nm in wavelength, but the distance between particles in the lower atmosphere is much, much smaller than that (around 3 nm).
Because of this, the light given off by any electron will almost always destructively interfere with light form another electron, because the waves will be out of sync with each other.
In the upper atmosphere though, where the distance between particles is much higher, not so many waves will destructively interfere with each other.
So all of this explains why the sky (or the "upper sky" if that makes any sense...) appears blue, but let's take a look at some other interesting consequences of Rayleigh scattering.
Other examples of Rayleigh scattering
Firstly, let's explain why the Sun appears yellow, because in reality, the Sun is actually white.
We see the colour white when our eyes receive an even distribution of every colour, and the for the most part, the Sun gives off even quantities of all the colours.
However, since most of the blue light has been scattered across the upper atmosphere, the light that we see from the Sun is void of much of its blue light, which makes the Sun appear yellow(ish).
Hopefully that's pretty straight forward, so now I'll follow-through on the promise I made earlier of explaining why the sky doesn't then appear purple if purple light is scattered more than blue.
And the answer is weirdly boring and interesting at the same time...
It might come as a surprise to you (it definitely did for me) but more often than not, what you think is purple isn't actually true purple, or "monochromatic purple" as it's scientifically called ("mono" meaning one and "chromatic" referring to light).
I know that sounds strange, but what we generally see as purple is just a combination of red and blue.
Purple light makes up a very small portion of the visible light spectrum, and it's a simple as we're just not that good at seeing it.
It borders with the next form of radiation that our eyes can't see at all (UV), and actually a lot of people can't even see monochromatic purple themselves.
So that's why the sky doesn't appear purple - purple light get's scattered more than blue, so the light is definitely there, but we just suck at seeing it, because it's true, monochromatic purple and not just a blend of blue and red.
Maybe the sky does appear purple to some animals!
Another interesting application of Rayleigh scattering would be why the Sun appears red at sunset, which is very similar to the previous answer of why it appears yellow at daytime, just on a more extreme scale.
At the time of sunset, what's actually happening is that the Earth has been rotated such that you're about to be facing completely away from the Sun, which is why it seems to drop below the horizon.
Hopefully you can image it pretty well in your head, but at daytime you're facing straight towards the Sun, so it appears straight above you in the sky (if you're at the equator anyway), and as the ground/Earth rotates, the Sun appears to drift towards the horizon.
At daytime (12-1pm), you are facing straight towards the Sun, which means that the light coming from the Sun travels through the least amount of atmosphere at this time in any given day, which is why it feels the warmest.
Particles absorb light, leaving less energy to contact with your skin, and since temperature is just a measure of energy, the more particles between you and the source of light (in this case the Sun), the cooler it will feel.
At sunset, however, because there is now a significant angle (almost 90 degrees) between you and the Sun, the light has to travel through more atmosphere, which simply means that more Rayleigh scattering occurs.
Because of this, the light coming from the Sun appears to contain even less blue light than at daytime, as more of it has scattered across the sky.
White light with no blue at all appears red, where as white light with some blue light appears yellow.
Great, so now we know why the Sun appears red at sunset, but what about the sky?
Sometimes, the sky appears particularly red at sunset, other times it appears yellow/orange, and other times pink!
Well, we have to consider something from before - the density of the atmosphere.
Our atmosphere isn't a static, non-changing environment.
Our weather cycle drives drastic differences in air pressure/density which is responsible for drastic changes in weather, like heat waves, to thunderstorms, to hurricanes, to droughts etc...
Since Rayleigh scattering depends on the density of particles in our atmosphere, which itself is ever-changing, different colours can get scattered to varying degrees.
Remember how I said that the atmosphere was almost exclusively oxygen and nitrogen?
Well, it is (over 97%), but that 3% can make a huge difference depending on what it comprises of.
We've established that blue light (well, technically purple light) is scattered the most with oxygen and nitrogen atoms, because their electrons have vibrational frequencies that resonate with blue/purple light, but the same can't be said for all elements/compounds/mixtures.
High levels of water in humid conditions would cause different colours to get scattered, and the same principal can applied to pollen, dust etc... Resulting in different colours of sky.
And that's everything that you need to know!
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