An unanswered question
Where does light actually come from?
Maybe a question that you've asked yourself once before.
Maybe it's not... But it's definitely an interesting one!
Fortunately for us, it's actually a pretty simple one to get your head around of, especially if you're reading this with an already robust understanding of atomic structure, but I'll be going through that as well, so don't worry if not!
Anyway, let's get into it.
The most logical place to start would probably be with how the Sun generates light, since it's singlehandedly responsible for daylight here on Earth, but the same process can applied to a light bulb, LED, candle, hot piece of metal etc....
Using the particle model of the universe, which is the one that is currently accepted, we can say that everything around you (most things in the universe in fact) is made of particles that are themselves made of atoms.
I say most things, because there are a few exceptions that particularly stand out like black holes and neutron stars, where electrons and protons have been smashed together to form neutrons.
Looking more at the big picture though, physicists and astronomers have come to believe that the universe is more than likely full of a mysterious new form of matter that we call Dark Matter, and your guess is as good as any as to what dark matter's made from!
Atoms are responsible for the emission of light, but we can take this one step further and say that specifically the electrons inside of atoms are responsible for the emission of light.
See, atoms comprise of neutrons, protons, and electrons, which you'll often here as being referred to as subatomic particles, simply because they're smaller than atoms (sub-atomic).
Neutrons have a relative mass of 1 and no charge
Protons have a relative mass of 1 and a positive charge
Electrons have a relative mass of 0 and a negative charge.
Note that the mass of an electron isn't 0, but if we say that the mass of a neutron is 1, then the mass of an electron relative to a neutron would near enough be 0.
Protons and neutrons group together under the "strong nuclear force", which is one of the 4 fundamental forces as prescribed by QM (Quantum Mechanics), along with gravity, electromagnetism and the weak nuclear force.
Together, they form the nucleus of an atom.
Something good to bear in mind at this point, although you don't necessarily need to know it for this article, is that we currently understand that protons and neutrons themselves can broken down further into something that you might have heard of before - quarks.
There a few different types of quarks that come together in different arrangements to form protons and neutrons, among a few other subatomic particles that I won't go into detail here. Electrons, on the other hand, are considered a lepton - a fundamental particle that cannot be broken down further.
Meanwhile, electrons "orbit" around this nucleus in different shells that each have different energy levels.
This is the most basic understanding of it, and while it's not technically all that accurate, in that electrons don't orbit around the nucleus like the planets do around the Sun, an understanding beyond it isn't necessarily needed, although why not!
QM is a solution to a problem that arises from the historical notion using classical Newtonian mechanics that electrons orbit around their relative nuclei.
This simply cannot be true, and I'll give a short explanation of why below for anyone interested.
A lot of people often question why electrons don't simply fall into the nucleus, since they're negatively charged and the nucleus is positively charged, so what's keeping them apart?
Well, this too can be explained by classical mechanics, although we'll come on to where it goes wrong in a minute.
The reason why electrons still wouldn't just fall into the nucleus if they followed Newton's laws of motion is because they're constantly accelerating.
If electrons obit around their nuclei like planets do around stars, then they would be constantly accelerating as they change direction.
While we usually associate acceleration with a change in speed, it's actually a change in velocity, and it might come as a surprise to you that the two aren't one and the same, even though they're used interchangeably in conversation.
Speed is what we call a scalar quantity, meaning that it has no definite direction.
Velocity, on the other hand, is a vector quantity, meaning that its direction is specified.
Acceleration is a change in velocity, not speed, which can either come from slowing down, speeding up, AND/OR changing direction.
Therefore, we can say that even though the electrons might not be gaining or losing any speed, they're still constantly accelerating as orbiting around nuclei involves constantly change direction, even if just slightly.
The same principal is what keeps the planets from just falling into the Sun - they're constantly accelerating as they change direction.
While this might seem pretty harmless at first, this concept produces an outcome that simply cannot be correct.
Larmor's work on moving charges tells us that when charged particles (such as electrons) accelerate, they radiate energy.
This energy comes from the conversion of their own kinetic energy, and hence their velocity decreases over time.
Larmor's power formula tells us that over time, the radius of an electron's orbit would eventually reduce to zero...
So yeah... probably not what's happening!
But what's the solution?
Well, that's a subject for a different day, but briefly it's just that electrons aren't classical particles - they're quantum particles, or more specifically waves.
They don't obey classic Newtonian mechanics, and instead follow the principals laid out by quantum theory, or so physicists think...
Anyway, back to the origins of light!
The most important part to understand here is that electrons are bound to very specific energy levels, and they can't exist between them.
The different bands or clouds of energy levels are separated by relatively massive differences. Highlighting the word "relatively" here, because obviously they're extremely tiny in terms of actual distance, as we're talking about atoms here, but compared to the size of neutrons, electrons, and protons, these gaps are absolutely huge.
They're so vast in fact that an atom can actually be said to be 99.9999999999996% empty space! If you're interested in knowing how that could possible be true, then give this article a read, as it's far too off-topic for this one!
These gaps between the electron shells and the nucleus account for 99.99% of the volume of an atom, and each individual shell has its own specific energy level.
Remember, electrons cannot exist between these levels - that's a very important part to understand!
Not all electrons can bunch themselves in the same energy level though, as they naturally repel each other via electromagnetic forces (electrons are negatively charged), so they're forced to fill out these different shells, or energy levels, starting from closest to the nucleus, which is the lowest energy level.
They start from closest to the nucleus, because electrons don't like being energised - they naturally want to exist in the lowest possible energy state.
When these electrons receive energy, for example by absorbing infrared radiation (heat), as is the case inside the Sun and in a lot of instances on Earth, they become "excited".
But remember - electrons don't like being in these "excited" states, and their easiest way out is to somehow release this energy, so that they can drop back to their original energy level.
How do they do this?
By releasing radiation of a very specific energy, such that they can drop back to one of those very specific energy levels that we've talked about. Apologies if i'm going on about it too much now, but it's absolutely fundamental to why different colours of light can be emitted, and even different types of radiation all together.
Something else that's key to consider at this point is that all the different elements in this universe (that we know of at least) are made of atoms that each have a specific amount of protons, neutrons and electrons unique to that element.
For example, oxygen is an element with one oxygen atom having 8 protons, 8 neutrons and 8 electrons.
The significance of this is that energy levels can vary from atom to atom, so electrons will have to emit an amount of energy that's specific to not only the energy level, but the atom as a whole.
These unique energy levels are responsible for the emission of different wavelengths of light, and hence different colours.
If you're not familiar with the Electromagnetic spectrum, and the concept of associating wavelengths with colours is confusing, then I'd recommend checking out this article.
To summarise so far, an electron will become excited if it absorbs energy from, for example infrared radiation, and will release a very specific amount of that energy in order to drop back down to a lower energy level, as electrons don't like to be in these excited states.
However, only a certain number of electrons can occupy a given energy level, so they can only drop down to the lowest possible energy state.
By the way, 1 packet of energy that oscillates up and down throughout space-time is referred to as a photon of light.
We can say that photons will oscillate up and down at different rates, depending on how much energy they have, to form different categories of what we describe as radiation. Just thought I'd mention this so that we don't get confused between different words!
Because these different energy levels are unique to every element in the universe, the energy that's released will also be unique to a particular element, since electrons will be releasing an exact amount of energy to drop them to these specific energy levels, as they can't exist somewhere in between - they must occupy a certain energy level.
An electron will release this energy in the form of electromagnetic radiation, and the amount of energy released will determine the wavelength and frequency of that radiation, and hence its position along the electromagnetic spectrum.
Visible light (ie the light that we can see), comprises of a small range of wavelengths across this spectrum, and each wavelength of this section has a different colour associated with it.
This is exactly how different elements produce different colours of light!
For example, if you heat copper up high enough, you'll begin to provide the electrons that make up the atoms of that copper with enough infrared radiation (heat), such that they're forced to release energy of a specific wavelength, which in the case of copper, is what our brains interpret as the colour green.
Remember, it's just a wavelength of light, and some animals won't even be able to see it.
The theory of evolution suggests that eagles, for example, have adapted to only see infrared radiation, in order to physically be able to identify the heat from their prey.
On the other hand, as humans, our eyes/brains have adapted to see the visible spectrum, so that we can recognise and differentiate between the light from fruits, berries, melons etc, as we rely on them for food and need to be able to easily identify them amongst branches and leaves.
The key takeaway from this is that visible light is just radiation of a specific range of wavelengths, just like UV, infrared and so on...
Radiation is everywhere, and everything around you is constantly releasing it in the cooling process, but the energy that's released isn't high enough for you to see it with your own eyes, not until things get really hot anyway!
As a side note to be provide some clarification, the energy of an electromagnetic wave (radiation) is proportional to its frequency (how many waves it completes per second), and is given by the famous equation e = hf, where:
e = energy
f = frequency
h = Planck's constant
For example, if you heat metals up high enough, they being to cool by releasing energy that's high enough for you to be able to see (visible light, hence why they appear to glow) but until then, it's mostly infrared.
Another principle to understand here is that atoms contain a vast amount of energy levels, and an electron can exist in any of them.
Depending on how much energy electrons absorb, and what shells are available to them (remember that there's a maximum amount of electrons for each shell), the amount of energy that they release can vary.
In addition, you're average piece of metal like copper is going to contain billions upon billions.... of copper atoms, each containing 29 electrons (in the case of copper that is) and each absorbing different amounts of energy.
Hence, they each drop down to different energy levels - energy levels that are unique to a copper atom, but different levels that exist in that atom.
Therefore, it's usually the case that a spectrum of light is released (different colours), as opposed to just one wavelength of light. There is, however, usually one wavelength of light that stands out in terms of intensity.
Take hydrogen that makes up 98% of the Sun for example.
There are trillions of trillions of trillions..... of hydrogen atoms that are releasing energy in the form of photons of radiation, and each hydrogen atom contains 1 electron that would absorb different amounts of energy depending on a whole host of factors.
Therefore, different wavelengths of light will be emitted for each atom, but there will be a dominant one amongst these. In this case, violet, blue, red and green are emitted, but the most intense is red.
As interesting as this all is, there's a far bigger picture to look at here other than distinguishing between colours with our eyes, because, quite frankly, our eyes don't do the best job of it..
I suppose the theory of evolution would suggest that there's no need to for the sake of our survival - we just need to see red, blue, green etc, and don't care about whether it's the specific wavelength of sodium, helium, nitrogen or whatever.
We can and have been doing so for decades create machinery that recognises very specific wavelengths of light.
We can burn every element here on Earth, use machinery to record the exact wavelengths of light emitted, and then we can use this data to refer to as we explore the universe!
This is how we know (or think we know) what the Sun, the planets and even stars/planets beyond our solar system are made of - we take the light that they're emitting, and use machinery to compare those spectrums of different frequencies and wavelengths of radiation to the spectrums that we have recorded for the all elements here on Earth.
Since an element is an element, no matter where in the universe it is, we can determine exactly what the star/planet at question comprises of, simply by looking for the spectrum that it matches to!
For example, when we look at the light from the Sun, we see that it is near enough identical to the spectrum that's released by hydrogen, so we can say that it must be almost exclusively made from hydrogen.
It's an incredible tool at our disposal, and it's called "light spectrometry" if you want to look it up and read more about it.
However, that probably does it for this article, since we've covered how light is emitted by electrons via the absorption and hence the following release of energy.
One other idea to understand though to finish this article off, is that electrons are doing this near enough all the time... I did briefly mention this earlier, but I feel like it's an important idea to get your head around of.
Every time an electron absorbs energy, it wants to get rid of it.
You don't have to burn gases or pieces of metal etc for electrons to release energy, as they're doing it every time they absorb the energy from the Sun, or from a light bulb etc, but the amount of energy from this will be too small for us to recognise, as it corresponds to a wavelength that is outside the range of visible light.
If you ever get the chance to use an infrared camera, then you'll see that people and objects are constantly releasing light from the exact process that we've talked about, but it's not high enough in energy for our eyes/brains to interpret.
Objects all around you are cooling down, perhaps from say 30 degrees Celsius, to 28.
That difference requires electrons to release photons of light that aren't high enough in energy for you and I to be able to see, but they're still there none-the-less!
It's only when things get really hot do they release energy in the form of visible light radiation, hence why you have to burn gases with flames and melt metals down to get them hot enough to glow.
Hopefully this was an interesting read!
I have a load of articles like this here at Expansive that go over the kind of questions that you might ask yourself, like "why is the sky blue", so definitely stick around if you're interested in those!
Thanks for reading!