Wow.. This is a big one. Light. It effects every aspect of our daily lives. You’re using it right now to read this blog post. We’ve become really good at producing and manipulating light. But, after doing an episode of Epic a while back called, “Lies you’ve been told.” I received a lot of.. Let’s call them, “inquiries” about ‘primary colors’. After trying to answer some of these questions, it quickly became apparent to me that not everyone really understands what light is made of. So, I thought it would try and shed some light on the subject. (See what I did there? I said ‘shed some li-‘ Oh, forget it.. Moving on..)
The first step towards understanding light is dismissing the casual observation of it. What one “sees” is the effects of the light interacting with the chemical and biological receptors in the eyes. Based on these interactions, the brain creates a rather subjective construct of the world around us.
Light is energy. The universe is full of it. Energy tends to move from one place to another. Light is one of the ways through which this happens. But how does it go from being thermal energy (heat) or mechanical energy (kinetic) to light? To answer these questions, let’s start by taking a look at the humble atom. The atom in our example is a hydrogen atom. It has one proton in the middle (the nucleus) and one electron zipping around it. That electron has a certain amount of kinetic energy which keeps it zipping around the nucleus and never actually coming into contact with it. The electron ‘wants’ to remain at a specific distance from the nucleus. And, in remaining at that distance, it must maintain a certain amount of kinetic energy. Less kinetic energy and it will move closer to the nucleus. More and it will move further away.
There are a few questions that you might have at this point:
1. Why did I use the word ‘zip’ and not ‘orbit’?
A: ‘Orbit’ implies an elliptical path similar to a planet revolving around a star. An electrons movement is chaotic and unpredictable. It’s also wicked fast. As a matter of fact, if you were to observe it with the naked eye (which you can’t.. So don’t try). The electron would appear as if it were a semi transparent shell encompassing the nucleus.
2. Where did the energy keeping that electron zipping around come from?
A: Somewhere else. Energy cannot be created nor destroyed, only moved around. Where did the energy come from originally? Best guess: The Big Bang. Since the Big Bang also started the clock running on time itself, you could say that the energy has literally always been around somewhere.
3. Why does an electron want to stay a ‘specific’ distance from the nucleus?
Loaded question. Electrons like to stay at very specific distances from the parent nucleus and will play some pretty neat tricks to do so. We call these discrete distances, “shells ” and there is more than one. Each shell has its own rules regarding the amount of kinetic energy required by the electron to get into them, and how many electrons can occupy them simultaneously. I won’t go into more detail about it, but to answer the question of why these shells exist at all, my best answer is, “I have no flipping clue.”
Ok. So now that we know about the atom, let’s play a little thought experiment. Let’s make ourselves super powerful and shrink ourselves down to the sub-atomic size. Now, let’s somehow track down that electron, (forgetting about what Heisenberg has taught us) and give it a big kick. The result would be the electron gaining a bit more (if we kicked it properly) kinetic energy. Like a tether ball spinning around a pole (without, you know.. Wrapping itself around it..), the extra kick would cause the electron to zip around a bit further away from the nucleus.
Remember that those electrons don’t like being anywhere else other than in those specific ‘shells’? Yeah, that makes things a bit more complicated.. We have to give that electron just enough kick to move it out to the next shell. Now we discover something else about electrons and shells. Since electrons are attracted to their parent nucleus, they will try to get as close as possible. So, if Mr. electron is in one shell and there’s another shell closer to the nucleus with room for it, it’ll try to jump down into that one. And that’s exactly what this electron is going to do. So, after getting a kick to a higher shell, that electron is going just to shake it off and jump right back down to the lowest shell it can get into.
Remember how energy cannot be created or destroyed? We seem to have violated that principle here. Each shell requires a certain amount of energy for an electron to remain in it. Let’s say that the closest shell (we’ll call this shell “K”) requires 1 unit of kinetic energy for an electron to be in it. The shell above it (call that one.. “L”) requires 4 units of energy for an electron to remain in it. Well, by this logic, we would have had to have added exactly 3 units of energy into the electron (by kicking it) for that electron to have moved from K to L. But what happened when the electron jumped back?
Turns out those electrons are pretty sneaky. While we weren’t looking (another Heisenberg joke), the electron took those 3 units of energy and threw them away! Right out of the atom! Now the question is, how can you take something like ‘energy’ and ‘throw it away’. Energy is not a ‘thing’ you can just toss, is it? Turns out.. It is! The electron scrunched up that extra energy into a little ball and threw it.
Now to call it a ‘ball’ is a little bit of a misnomer. We imagine it as a ball. But, if it were an actual ball, it would have to come from somewhere. Which would mean that the electron would have only a limited supply. If it had only a limited supply, then it would eventually run out (after being kicked up to the L shell and falling back down to the K shell repeatedly) .
Again, that sneaky little electron plays a little trick on us. The ‘ball’ has no mass. It literally isn’t a thing because it has no mass and therefore is not made up of matter (I’ll get to Higgs field interactions another day). So, the electron can throw as many as it wants. The photon is a packet of pure energy which is accelerated away from the atom. Accelerating something that has zero mass comes with consequences. To understand them, let’s take a look at a very old equation:
Force = Mass x Acceleration
Rearranged, this becomes Acceleration = Force / Mass
The formula is simple, if you were to apply a force of 2 Newtons to a 4 kilogram mass, it would accelerate at 0.5 meters per second squared. (in other words, the velocity of the mass would increase by 0.5 m/s every second). Now, let’s reduce the mass to say, 1 kilogram. Now the same force of 2 newtons would accelerate it at a rate of 2 meters per second per second. If we reduce the mass to 0.1 kilograms, the acceleration would increase to 20 m/s per second. What happens if we make the number infinitely small? The 2 newton force would accelerate it at infinite m/s per second. In fact, once we hit zero mass, any force applied, no matter how small, would instantaneously accelerate the mass-less particle to an infinite velocity (instantaneously covering an infinite distance and therefore disappearing from the universe).
Since a photon has no mass, it is instantly accelerated to an infinite speed. But, as it turns out, our universe has a built in speed limit. Yes, the photon will accelerate infinitely quickly, but it will stop accelerating once it hits 299,792,458 meters per second. Why this particular speed and not some other speed? I don’t have an answer to that. It’s just the way our universe exists, as far as I know. Hopefully a future article might cover this
Good so far? Good. Let’s throw something else into the mix. Different energies. Remember how we said that the electron needed to kick out a specific amount of energy to go from one shell to another? Well, let’s extend that example. Let’s say that there are three different shells. We’ll call them “K”, “L”, and “M”. K is the closest to the nucleus (and therefore requires the least amount of energy to be in) and M is the furthest away (and therefore requires the most amount of energy to be in). K requires 1 unit of energy to be in, L requires 4 units, and M requires 6 units (these numbers are totally made up, by the way). We give the electron (which start in K) a 5 energy unit ‘kick’ so that it jumps all the way past L and into M. Once again, it begins to fall. The first fall occurs between M and L and produces a 2 energy unit photon. The second fall occurs between L and K and produces a 3 energy unit photon.
How does the first photon differ from the second? If we were talking about something with actual mass (like.. a baseball) we could say that the second photon was thrown a bit harder than the first, giving it an extra unit of energy. But photons all travel at the mysterious 299,792,458 m/s speed limit. They can’t go any faster (or slower).
Again.. Another sneaky little trick. This time played by the photon itself. Since the photon has no mass, it is not bound by all those pesky mass-related rules. You know what else carries energy and doesn’t have to obey mass-related rules? Waves. Two energy waves can carry different energies and travel at the same speed. They do this by changing their frequency (or wavelength) Putting more energy into a wave simply means it will move up and down quicker, not that it will propagate outwards faster. And so we have the classic duality of the photon. It carries energy like a wave, but moves along in a neat little bundle like a particle.
Now that we know all this interesting information about photons, what does this have to do with light? Well, light is made up of photons. We interpret photons as follows: The energy (or frequency or wavelength) of the photon defines the color. The number of photons striking the eye in a given time period defines the ‘brightness’. Our eyes can only assign colors to photons between the frequencies of 400 and 789 terahertz. Below or above that, and the eye cannot ‘see’ them.
So, what colors can a photon be? Why are there no pink or brown photons? What does this have to do with ‘additive’ and ‘subtractive’ colors? How do colors get reflected or absorbed? How do light bulbs generate light? What’s all this business of primary colors? We’ll take a look at this and more in upcoming blog posts.