The Saga of Light, Part 1: Electromagnetic… RADIATION!!!

Did you know Jean Claude Van Damme was almost the Predator? True story. And also, interestingly, if he had stayed on the film he would have been the only member of the main cast to not run for Governor. That’s not true, I made that up. Actually… weeeeeell… Where’s your sense of civic duty Shane Black? You’re ruining everything!

I bring up “Predator” because it’s pretty relevant to what I want to talk about over this series of blog posts. In fact I want to talk about a lot of things. I want to talk about some pretty out-there things like what exactly is the predator seeing and why is it useful; how to make a medical tricorder (a la Star Trek); how to see through solid objects; how pit vipers can (partially) see in the dark and how we can determine the composition of distant stars from the safety of our homes. (Yay for lazy science!)

On top of that I also want to talk about how a number of technologies work, like laser thermometers, neon signs, microwave ovens and incandescent lightbulbs. I want to talk about how the heck this:

Arecibo_Observatory
[Source: https://en.wikipedia.org/wiki/File:The_Arecibo_Observatory_20151101114231-0_8e7cc_c7a44aca_orig.jpg]

is actually a telescope… And where Sean Bean died at the end of Goldeneye… I want to talk about why black-lights make one’s (hypothetical) dandruff problem suuuppppeeerrr obvious.

Finally, I want to talk about some of the everyday physics that govern our lives whose awesomeness is maybe not always appreciated. Like how you and everyone you’ve ever loved is glowing. All the time, every day, every night, every moment. I wanna talk about why humans see what we see and why we evolved that way. I wanna talk about what makes silver, ya know, “silver” coloured and white, “white” coloured and how something can simultaneously be black and shiny, which at first glance would seem to be impossible. I wanna talk about why you need to wear sunscreen and what happens if you don’t. And I even wanna talk about some very simple things like how light reflects; what light actually is; what it does; and many of the ways it can be created. I’ll also be throwing in a little bit of quantum physics for good measure, because why not.

So, that seems like a lot of stuff. But really, I want to just talk about one thing: *electromagnetic radiation* and its many aspects. From that one topic all of the above will follow.

It’s also my goal to set up these series of posts as something I can constantly refer back to in the future. I want them to be a spring-board for future, crazier, more advanced discussions like how light can be used to hold things in place like a tractor beam; or push it around like a propulsion system for spacecrafts; or how light can even be used to freeze things; how you can twist light and I’m sure much, much more. And I can only do that if we have these key fundamentals in place.

So in light of my goals (puns!) I do err on the side of giving more complete discussions in many places. It may make things a bit less breezy but hopefully it will pay off in the long run in terms of giving a deeper and richer knowledge of physics that we can build off of. I will also warn that I’ve back-loaded a number of the topic you might want to hear about most, like medical tricorders and predator vision. But in order to talk about those we need to really understand the fundamentals; we gotta eat our vegetables before we can get to dessert. (I mean, I hear that’s how people with a modicum of self-restraint go about things, at least) So keep in mind if things are starting a little basic that with each new bit of understanding we add to our utility belt the more we can focus on using what we’ve learned.

Or to put things more bluntly: the more we learn the more fun we can have with it. In my books, any post that let’s me talk about Predator, Star Trek and Contact – all in one go – is okay-by-me. We’re going to really get into some of the key aspects. But upfront here’s the path of posts we’re going to take, with each post being released (hopefully) every two weeks or so:

Part 1: Electromagnetic… RADIATION!!! (topics: electromagnetic radiation, Bremsstrahlung, cyclotron emission, black-body radiation)
Part 2: ALL The Colors of Light (topics: the electromagnetic spectrum, radio, microwaves)
Part 3: Bouncing Light: Knowing Mirrors from White Cotton (topics: reflection of light, sub-surface scattering)
Part 4: How to See Through Things (topics: absorption, reflection, emission spectra, origins of color)
Part 5: The Need For Photons (topics: the photoelectric effect, atomic spectra)
Part 6: How to Make Things Glow (topics: impact ionization, neon signs, fluorescence)
Part 7: Tricorders, Microwaves, Sunscreen and the Ozone Layer (topics: nuff said)
Part 8: How Does Predator-Vision Work? (topics:black-body radiation, predator vision, pit vipers)

So, let’s get started.

Electromagnetic Radiation!!! (Insert dramatic musical cue)

One day I want to make a whole set of posts just about the term “radiation”. What the term means; the types of radiation; the dangers it presents; the benefits it presents; the extreme amount of misinformation surrounding the term; the physics; the technology; all of it.

But these posts are not those posts.

On top of this, culturally, we’re often primed to make this immediate gut association between the word “RADIATION” and “BAD!!!”, especially for those who grew up during the cold war. What type of radiation? What level? What energies? Doesn’t matter. “RADIATION” equals “BAD”.

In reality, radiation can mean a lot of things. It just means something which radiates. Even the everyday light coming from a regular lightbulb or a camp-fire is a form of radiation. It’s called electromagnetic radiation. It’s that kind of radiation that’s our focus here.

Now it’s probably not too shocking (puns!) to find out that in order to learn about “electro” “magnetic” radiation we are going to have to learn just a bit about “electromagnetism”. However, electromagnetism is a topic that is equal parts complex and non-intuitive, at least relative to everyday experiences. Our ape brains have a clear intuition for things like “mechanics”; that is, we have a sense of the trajectory a ball will take when we throw it, what happens when a heavy object slides down a hill or when a car makes a tight turn. However, our ape brains have no intuition whatsoever for what electric fields surround an electrical-current-carrying wire, or how electrical charges re-distribute themselves in a conductor. In fact, the only real intuition our ape brains provide us is stuff like “lightning pretty, but very bad. Scary. Stay away!”

But don’t worry, I have no intention of tossing us head-first down a rabbit-hole of complicated physics. However, as a result, in order to keep things direct and simple, I am going to simply state a number of things that are “true” about electric and magnetic fields and how they behave. We’re not going to focus on the “whys” or “hows” we just need some concrete facts in place so we can move on. So, if you feel unsure and perhaps puzzled by some of the concepts we’re going to lay down, that’s totally fine. Consider these the Coles Notes, or CliffsNotes for you Americans, (do those still exist?) for a book you’ve never read and you weren’t expected to have read. At the end I will summarize the key points we need moving forward and if you get lost just remember that those key points are all we need and you can simply take them as statements-by-fiat.

We Gonna Rock Down To… Electric Avenue

In our universe, the fundamental particles that make up all matter have a property called “charge”. Some, like the electron and the muon, have a negative charge. Others, like protons and positrons, have a positive charge. (Strictly speaking, a proton isn’t a “fundamental particle” but is actually made up of smaller particles, called quarks, but we really don’t care about that level of detail.) Some, like neutrinos and neutrons (also not fundamental) have no charge at all.

Why? How? Like I said – for these posts, at least – we don’t care. It is simply something that is “true”.

If a particle has “charge” then that means that it produces all around it an electric field. Furthermore, if a charge is placed in a spot where there is already an electric field due to some other charge, the placed charge will feel a force towards the other charge of either repulsion or attraction; repulsion if both charges are of the same type (i.e. both positive charges or both negative charges) or attraction if they are of opposite type.

We typically show the electric field that surrounds a charge using a diagram like this:

E_Field_Around_Charge

Such diagrams are meant to show two things: 1) the direction a small positive (by convention) “test” charge would be pushed/pulled due to the electric field if it were place at a given location (i.e. that’s what the directions of the arrows at different spots mean, the direction a charge placed there would be pushed), and 2) that the strength of this field gets weaker the farther you are from the charge emitting the field (i.e. that’s what the length of the arrows mean, shorter arrows mean weaker field strength at that point).

The specific way the strength of an electric field diminishes with distance from a charge isn’t really important to us, but for you keeners it’s called an “inverse square law”. This means that if I double my distance from a charge, the electric field I detect will quarter. If I quadruple the distance it will… whatever the English word for “1/16th” is. But again, the specifics aren’t so important.

So the key take-away for electric fields is that if I have a charged particle just sitting there, there will be an electric field surrounding it. If I used some “electric-field-o-meter” to detect this field I’d find, firstly, that at any given point the electric field has a direction to it, like a little arrow, and secondly that the strength of the field will diminish as I get farther and farther from the charge.

The specific way that the strength decreases is something like this:

E_Inverse_Square2

What is crucial about this is that once I’m far enough away from a given charge, I won’t be able to detect the electric field with my detector. It will be like it doesn’t exist at all. (I wish I could say the same for the career of Pauly Shore *Insert more recent reference here*).

One last important point is that the electric fields of positive and negative charges point in opposite directions. So in my arrow diagram above, I’ve shown the field around a positive charge and the arrows then point away from the charge because, by convention, they are meant to show how a tiny positive charge would be pushed if placed at a given spot. If I were instead to make a diagram for a negative charge it would look identical except all arrows would be flipped by 180 degrees because a positive “test” charge would be attracted towards the negative charge.

Because opposite charges produce fields of opposite orientation, the combined electric fields of opposite charges can act to cancel one another. In fact, if I have a equal amounts of positive and negative charge placed exactly right on top of one another, I would not be able to detect an electric field at any distance whatsoever because for every arrow pointing one way due to the positive charge there would be an arrow equal in magnitude but in the exact opposite direction added to it by the negative charge and thus cancelling it.

This cancellation is actually the “default case” in everyday life as matter is made of atoms and atoms have a positively charged atomic nucleus encircled by negatively charged electrons. Although the electrons and nuclei are in physically different locations, with the electrons being outside the nucleus in, so-called, “orbital states”, the average location of the electrons and the nuclei fall on top of each other. By definition an atom has equal numbers of positive charges in its nucleus and negative charges due to its electrons (if there aren’t equal numbers then it’s called an “ion” rather than an “atom”).

The key point is then that atoms are made of charges but they typically, on average, have no net electric field and are, on average, neutrally charged as their charges cancel.

Let’s now take a look at the other half of “electromagnetism”: magnetism.

Optional Note for Advanced Readers

Of course I’m intentionally avoiding a discussion of electric dipoles here, where two opposite charges that initially cancel can be forced to offset slightly from one another – by say an external electric field – producing a anisotropic force proportional to 1/r3 rather than the isotropic 1/r2. But, the dipole oscillations of atoms will not be of enough importance in this specific set of posts to bother with the complication of discussing them.

Breakin’ 2: Magnetic Boogaloo

Now, when I mention magnets, it’s likely that what comes to mind are permanent magnets – like fridge magnets, or my winning personality – where the magnetism is seemingly some intrinsic property of the object itself, much like charge. We could get lost in a pretty advanced and nuanced discussion as to whether this notion of “intrinsic magnetism” really holds up in “Physics Court”, but what we’re concerned with here is a much more natural origin of magnetism: specifically, if an electrical charge is set into motion then a magnetic field results.

Conceptually, magnetic fields are exactly like electric field in the sense that you can think of them as little arrows that permeate the space around a “magnetism source” (be it moving charge or fridge magnet). However, the way and direction those arrows point is quite a bit different than electric fields and, to be totally honest, pretty hard to get an intuition for. Luckily, we really don’t care about this complexity-headache here, but just to at least show it once, if I have an electric charge which is moving with a constant velocity I will find that, as a result of this motion, a magnetic field will emerge around the charge that: 1) curls around the direction of its motion, and 2) diminishes in strength the farther you are away from the charge. I’ve drawn a little diagram here:

B_Around_Single_Charge

But I really want to re-iterate that if this looks super-complicated, to not worry as we don’t really care what direction the arrows point we’re just concerned with the second part: the strength of magnetic fields decrease with distance from the source. In fact, they decrease in strength in exactly the same way as electric fields (i.e. inverse square law) when the magnetic field is due to a moving charge (with constant velocity). (Just an FYI, it’s actually not of this form for a permanent magnet – it’s a weaker inverse cube law – but like I said, we don’t care about such magnets.) So just like our electric fields, if I have a “magnetic-field-o-meter” and I go far enough away from some charge moving with a constant velocity, I won’t be able to detect the field at all.

What is absolutely crucial here is that magnetism is not a distinct phenomenon from our charges and electric fields, but rather magnetism is the result of electrical charges in motion. In the case of a single charged particle moving with constant velocity, I will have both an electric field, since the particle is charged, and a magnetic field, because it is moving. However, this situation is a little complicated to think about because in addition to our “inverse square” decrease in field strength with distance I also have an increase/decrease simply due to the fact that the charge may be heading towards or away from me. But, the key truth is that magnetism really is related to the motion, and this can be clearly seen in the very common scenario of an electrical current, where the fields are much simpler to understand.

Electrical Current

If I have a regular piece of metal (like a disconnected wire), it has no net electrical charge. Of course you might try and be difficult and point out that you can give a piece of metal an excess of charge through something like static electricity – i.e. forcing some small amount of electrons from one object to another, creating an excess of one type of charge within each object – but this isn’t the case we’re interested in. If I then take this neutral metal wire and hook both ends of it up to a battery I will get an electrical current.

By definition, an electrical current is a net motion of charge. Now, the internet is filled with many, many very wrong explanations of what exactly is happening to our electrons and atoms in a wire when an electrical current is moving through it. The real answer is quite complicated and perhaps a story for another time. However, we really don’t need to concern ourselves with the microscopic specifics. All that matters is that at all points along a wire I have this steady net motion of charge towards the direction of the current flow. We don’t care what particle is specifically doing what in what way, just that if I conceptually made a sort of cross-sectional cut through a wire and kept track of the net amount of charge that passed through the plane of the cut it’d be a certain amount of charge per second; a current. Just like if I conceptually took a cross-section of a flowing water pipe and watched the rate of water flow.

What makes the case of an electrical current fundamentally different than the case of a moving electrical charge is really only one simple fact: in an electrical current, for a given point along a wire, even though charge is moving away from that point in-line with the current flow, it is also being replaced with new charge coming in from the other direction. I have drawn a very cartoony, very not-accurate conceptual diagram of this here:

B_w_Current

Do NOT take this as some serious statement of what is actually happening at a microscopic level, rather it just conceptually shows how at any given point, charge moving to the right is “replenished” by new charges coming in from the left and since the wire was neutral to begin with (i.e. for every negative charge there was a positive charge) the entire wire still remains neutral as the current progresses. In analogy you can think of a river or water pipe; you can easily have a strong current or flow in a river without the actual water level decreasing (i.e. you have a constant amount of water per unit volume and yet a net motion of water). Thus, if a river has a strong current it doesn’t imply that the river is “drying out” and being depleted of water because there’s constantly new water coming in upstream as water flows downstream.

Thus, in an electrical current, there is zero net charge, but still a non-zero motion of charge. In the case of a perfect, ideal wire you will have no electric field but still a magnetic field. More precisely, if I have a steady electrical current there is a very constant magnetic field surrounding the wire whose strength falls off in the same way as the magnetic field (i.e. inverse square law).

Optional Note for Advanced Readers

It’s worth point out that I’m actually totally lying right now about wires and their net charges, any REAL wire must actually have an associated electric field due to the accumulation of surface charges. Furthermore, this was known at least as far back as Oliver Heaviside. This is actually a reasonable fascinating topic (though fairly advance) and I refer the curious reader to the following references: American Journal of Physics 52, 1097 (1984),
Foundations of Physics, Vol . 29, No. 5, 1999 and even, if you dare, this one from J. D. Jackson himself: Surface charges on circuit wires and resistors play three roles. Those references are actually just the tip of the iceberg, just in the American Journal of Physics alone there are a dozen or so papers on the topic of the electric fields associated with wires of constant current. However, we can consider my discussion as focused on a hypothetical, magically-infinitely-thin wire.

Bringing it All Home

So, our key results have been that when I have a net charge, I have an electric field and when I have a net motion of that charge, I have a magnetic field. We also talked about how the situation in which you find magnetic fields take their simplest form is in a current-carrying wire where you have no net electric charge and yet there is a steady, constant and regular motion of charge along the wire.

We’ve now come to the big pill we have to swallow; the key take-way we’ve been working towards. However, I want to re-iterate that I have been intentionally brief in this whirlwind tour through electromagnetism and I am laying down the next result in the manner that I am simply telling you that it is true rather than as something that is in any way supposed to be “obvious” based on the very surface-level discussion we’ve just had. This isn’t supposed to be a natural and straightforward conclusion based on what we’ve just learned and to motivate it in that way would require a far, far, far more in-depth discussion. But let’s just lay it down.

I get an “electric field only” when I have a charge that is not moving. I get a “magnetic field only” when I have charges that are moving with constant speed (or constant current) but when there are equal numbers of opposite charges. In between those two extremes a very… different… type of field results.

Take a look at this animation I made:

GIF_Pulse

At the beginning of the animation, it is showing a charge at rest and the strength of the electric field and magnetic field as a function of distance from the charge. It’s just like the previous electric field diagram I made and we see that the electric field falls off in strength very quickly (according to an inverse square law) and since there is no motion of the charge the magnetic field is initially zero everywhere.

However, then I basically just give the charge a “flick” before returning it to being stationary. What do I mean by a “flick”? I literally move it up and down in a quick motion. Though don’t get confused by what I’m trying to show. The yellow charge is just there to illustrate what has happened and the charge is moving up and down, for real, in space. However, the rest of the graph shows field strength versus distance. That pulse is an undulation of the strength of the electric and magnetic fields that is moving in space. I think when people see graphs like this they intuitively try and interpret the y-axis as saying something about space, like as if the “ripple” is literally some vertical displacement of a string. No. We’re just looking along a straight line (the x-axis) and the y-axis tells us what an “electric-field-o-meter” or “magnetic-field-o-meter” would detect at that spot along the line at a given time.

More concretely, I suddenly give the charge an acceleration, causing it to speed up, then I give it a deceleration, slowing it down to a stop and then accelerate it again but in the opposite direction (so that I can send it back to where it started) and then decelerate as it comes back to a stop where it started. That may sound very complicated but really all that makes this different from our previous cases of a non-moving charge or of a charge moving with constant velocity is that the charge, while I’m flicking it, is accelerating and decelerating! This is what is causing that “pulse”, it’s the acceleration/deceleration.

There are three absolutely crucial things to take away from this animation. The first is that while the charge is accelerating or decelerating you get this pulse which includes both an electric field and magnetic field whose strength decreases much slower with distance than our regular “inverse square law” electric field. Thus, even at large distances, where the electric field due to the non-moving charge is negligibly small, the electric (and magnetic) fields due to this ripple are still clearly there. Thinking of things this way we can actually concretely understand what it means for an electromagnetic field to radiate: Even if I’m crazy far away from the source such that I really can’t detect its “static” field, if I can still detect something (i.e. the pulse) then it has radiated something towards me.

Optional Note for Advanced Readers

A reader with a background in physics may well take issue with the way I’m talking here, as they know that the intensity of electromagnetic radiation falls off like an inverse square law. So what am I on about? What does “radiate” mean? Well, the intensity of an electromagnetic field is related to the square of the field, i.e. energy is proportional to |E|^2 , where E is the electric field (not energy… to many E words!). So for a stationary charge with an electric field given by Coulomb’s law, which goes like E \propto 1/r^2 , the energy/intensity falls off like 1/r^4 ; where the radiative part of the EM field of an EM wave falls of like E \propto 1/r , thus the energy/intensity falls of like 1/r^2 . So we see that although the intensity of a radiating EM wave does eventually fall to zero at some far distance, a stationary charge does so at a power of 2 faster, which is a dramatic difference in long-distance behaviour. Thus, the precise definition of “radiating” is that the field falls off like 1/r and its energy falls of like 1/r^2 , rather than 1/r^2 and 1/r^4 respectively for a stationary charge.

The second important thing is that the propagating disturbance is composed of both electric and magnetic fields and the two travel together. What is not made clear by my animation is the relative direction of the electric and magnetic fields as the y-axis is simply “strength” of the field, but remember that fields also have a direction at a given point. It turns out that in a radiating electromagnetic field the electric and magnetic fields are always perpendicular to each other and perpendicular to their direction of motion. It doesn’t matter how I do the “flick”, the electric and magnetic fields will always point transverse to the direction of motion and perpendicular to each other. However, the electric and magnetic field, despite needing to be perpendicular, can still take any orientation in the plane perpendicular to the direction of motion. Now that’s a complicated sentence but here are two diagrams which should hopefully make it clear:

Polarization2

Polarization_Angle2

Thus, even if two radiating electromagnetic fields are both heading in the same direction, and even though their electric and magnetic fields are 90 degrees from each other, it is still possible for them to have, what is called, a different ”polarization angle”.

The final thing to take away from this animation is that this “radiating” “electromagnetic field” clearly has a speed with which it radiates. I can see its speed as it cruises from left to right in the animation. You might think, maybe, that the speed of that pulse might have something to do with the “flick” itself. Maybe if I flicked faster or harder the speed that the pulse traveled would change. It doesn’t. ALL electromagnetic “ripples” travel with the same speed.

So what is that speed? Well, it’s possible to exactly figure it out if we use the actual physical equations that underlie the physics of electromagnetism, which are called “Maxwell’s Equations”. Specifically, they gives us an exact number based on two experimental constants that are in the equations. I normally try to avoid mathematical detail but in this particular case let’s actually look at exactly what it ends up being. These Maxwell’s equations say that the speed of this electromagnetic radiation is 1 divided by the square root of something called the permittivity of free space times something called the permeability of free space.

\frac{1}{\sqrt{\mu_0 \epsilon_0}}

Now if you’ve never heard of the permeability or permittivity of free space I’ll forgive you. Their names alone are a bit eye-bleedingly obtuse. I mean, I’m pretty sure I fell asleep even saying the names. But basically the permittivity is an experimentally determined constant that tell us how big the electric field is that results from a given amount of charge, which is to say it’s an experimental constant that is just about electric fields only. Similarly, the permeability is basically an experimentally determined constant that tell us how strong the magnetic field is that results from a given amount of moving charge and is about magnetic fields only.

So this alone may not be so surprising, since we have here “electro” “magnetic” radiation and we find that its speed is related to fundamental properties of “electric fields” and “magnetic fields” simultaneously. So, no shocker. Again, who cares? Why is this interesting? Well, let’s just figure out what the actual speed of this wave is as a number. You can determine – through experiments – the actual values of these permitivitties and permeabilities… Or you could do what I did and Google them. (Minimum effort!) So I just plug them into my calculator, carry the one and…

Electromagnetic radiation moves at: 299,792,458 meters/second. So fast that it could go all the way from the Earth to the Sun and back in under 20 minutes.

But wait a minute, if you’re the kind of person who spends their free time looking at fundamental constants – and, I mean, who isn’t? – that speed might seem very, very familiar. 299,792 kilometers a second? (or about 670,000 miles per hour for you Americans) That’s the speed of light!

It doesn’t take a genius to put two and two together. (In your face Maxwell!) Light IS an electromagnetic “ripple”. That’s what it is. Or as Maxwell himself – who wrote down these “Maxwell’s equations” – commented while lecturing at King’s College in 1862:

We can scarcely avoid the conclusion that light consists in the transverse undulations of the same medium which is the cause of electric and magnetic phenomena

Though, a story for another day is what was wrong with Maxwell’s statement. Specifically, that there is no medium that light travels through. Though this discovery only began to become clear starting in 1887, 8 years after Maxwell had died. But a story for another day.

Mini-Summary

At this point we’re done our whirlwind tour of the physics of electromagnetism. The big take-away is that light, that thing that comes from the Sun and light-bulbs and candles IS electromagnetic radiation. Put another way, the term “electromagnetic radiation” is just another word for light. And thus, we see, light itself is an excitation, a ripple, of electromagnetic fields and all the light that has every hit our eyes owes its origin to some moment where an electrical charge accelerated or decelerated.

Just think of all the Hollywood movies and newspaper articles and New Age junk that talks about “electromagnetic radiation” and every time you see that now just replace it with the word “light” and see how it sounds. Now, I’m not saying that there aren’t many situations where we might be concerned about electromagnetic radiation, which we’ll come to understand much better through this series of posts, because LIGHT absolutely can be harmful. However, the amount of times you read an article that simply leaves a statement like “X emits electromagnetic radiation” without any clarification or specifics of why that is interesting or concerning and simply relies on the unfamiliarity of many of what the term means to generate some implied, but not explicitly state, “fear” is too damn high. “Technology X emits electromagnetic radiation!” is a shocking headline, “Technology X emits light” is probably not so much. Really? Your crystal hemp bracelet “redistributes” your body’s electromagnetic radiation? What does that even mean? It’s shiny?

For the rest of this post we’re going to talk a bit about a number of specific ways where charges end up accelerating and what ultimately produces the light that illuminates our daily lives (well, we’ll look at some origins here, and some others in later posts). However, since the last couple sections were fairly dense and… well… whirlwind-y, let’s just recount the key points.

Charges are something that exist and because of them there are electric fields. The strength of the electric field that results from such charges reduces dramatically with distance from the charge and becomes increasingly undetectable. Charges set into motion produce magnetic fields, regardless of whether there are net charges (again, just think of a river, the speed of flow of a river and the water level of a river are different properties). The strength of magnetic fields that result from constant motion – like a current or a charge moving with constant velocity – also reduces dramatically with distance in a manner identical to electric fields and thus are similarly undetectable at great distances.

Charges that undergo acceleration or deceleration – i.e. whose velocities undergo a change – emit or radiate a special kind of electromagnetic field that we called electromagnetic radiation. This electromagnetic radiation is a propagating “ripple” with both an electric and magnetic field component. The strength of electromagnetic radiation does not decrease nearly as fast as the electromagnetic fields due to stationary charges or charges moving with a constant velocity. “Light” is just another word for electromagnetic radiation and thus light originates from accelerating/decelerating electrical charges.

Sources of Light

In our next blog posts we will learn a lot more about the nature of electromagnetic radiation/light and its properties. However, for the rest of this post I want to hold off on asking what these electromagnetic ripples really “are”, in terms of their behavior and physics, and instead talk about where they come from. What I mean by this is that we’ll look at common scenarios where they are produced. Let’s first start with Bremsstrahlung. (See Page 2)

4 thoughts on “The Saga of Light, Part 1: Electromagnetic… RADIATION!!!

  1. Great blog post. Thanks for creating it. I have a somewhat related question. Why does changing magnetic field give rise to electrical current? I know it has to be so, it has been proven experimentally, and Maxwell’s equations describe it. However, what is happing in the conductor at the atomic level to make the electrons move in response to the conductor’s relative motion to the magnet? I have scoured the internet for a while but haven’t found a satisfactory answer, except that it is just so. I will appreciate any help in understanding this phenomenon. I am not studying this for an exam; I am just curious. I have been enjoying your YouTubes videos too. Thanks.

    Like

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