# The Liénard–Wiechert potentials and the solution for Maxwell’s equations

In my post on gauges and gauge transformations in electromagnetics, I mentioned the full and complete solution for Maxwell’s equations, using the electric and magnetic (vector) potential Φ and A. Feynman frames it nicely, so I should print it and put it on the kitchen door, so I can look at it everyday. 🙂

I should print the wave equation we derived in our previous post too. Hmm… Stupid question, perhaps, but why is there no wave equation above? I mean: in the previous post, we said the wave equation was the solution for Maxwell’s equation, didn’t we? The answer is simple, of course: the wave equation is a solution for waves originating from some source and traveling through free space, so that’s a special case. Here we have everything. Those integrals ‘sweep’ all over space, and so that’s real space, which is full of moving charges and so there’s waves everywhere. So the solution above is far more general and captures it all: it’s the potential at every point in space, and at every point in time, taking into account whatever else is there, moving or not moving. In fact, it is the general solution of Maxwell’s equations.

How do we find it? Well… I could copy Feynman’s 21st Lecture but I won’t do that. The solution is based on the formula for Φ and A for a small blob of charge, and then the formulas above just integrate over all of space. That solution for a small blob of charge, i.e. a point charge really, was first deduced in 1898, by a French engineer: Alfred-Marie Liénard. However, his equations did not get much attention, apparently, because a German physicist, Emil Johann Wiechert, worked on the same thing and found the very same equations just two years later. That’s why they are referred to as the Liénard-Wiechert potentials, so they both get credit for it, even if both of them worked it out independently. These are the equations:

Now, you may wonder why I am mentioning them, and you may also wonder how we get those integrals above, i.e. our general solution for Maxwell’s equations, from them. You can find the answer to your second question in Feynman’s 21st Lecture. 🙂 As for the first question, I mention them because one can derive two other formulas for E and B from them. It’s the formulas that Feynman uses in his first Volume, when studying light:

Now you’ll probably wonder how we can get these two equations from the Liénard-Wiechert potentials. They don’t look very similar, do they? No, they don’t. Frankly, I would like to give you the same answer as above, i.e. check it in Feynman’s 21st Lecture, but the truth is that the derivation is so long and tedious that even Feynman says one needs “a lot of paper and a lot of time” for that. So… Well… I’d suggest we just use all of those formulas and not worry too much about where they come from. If we can agree on that, we’re actually sort of finished with electromagnetism. All the chapters that follow Feynman’s 21st Lecture are applications indeed, so they do not add all that much to the core of the classical theory of electromagnetism.

So why did I write this post? Well… I am not sure. I guess I just wanted to sum things up for myself, so I can print it all out and put it on the kitchen door indeed. 🙂 Oh, and now that I think of it, I should add one more formula, and that’s the formula for spherical waves (as opposed to the plane waves we discussed in my previous post). It’s a very simple formula, and entirely what you’d expect to see:

The S function is the source function, and you can see that the formula is a Coulomb-like potential, but with the retarded argument. You’ll wonder: what is ψ? Is it E or B or what? Well… You can just substitute: ψ can be anything. Indeed, Feynman gives a very general solution for any type of spherical wave here. 🙂

So… That’s it, folks. That’s all there is to it. I hope you enjoyed it. 🙂

I talked about Feynman’s formula for electromagnetic radiation before, but it’s probably good to quickly re-explain it here. Note that it talks about the electric field only, as the magnetic field is so tiny and, in any case, if we have E then we can find B. So the formula is:

The geometry of the situation is depicted below. We have some charge q that, we assume, is moving through space, and so it creates some field E at point P. The er‘ vector is the unit vector from P to Q, so it points at the charge. Well… It points to where the charge was at the time just a little while ago, i.e. at the time t – r‘/c. Why? Well… We don’t know where q is right now, because the field needs some time travel, we don’t know q right now, i.e. q at time t. It might be anywhere. Perhaps it followed some weird trajectory during the time r‘/c, like the trajectory below.

So our er‘ vector moves as the charge moves, and so it will also have velocity and, likely, some acceleration, but what we measure for its velocity and acceleration, i.e. the d(er)/dt and d2(er)/dt2 in that Feynman equation, is also the retarded velocity and the retarded acceleration. But look at the terms in the equation. The first two terms have a 1/r’2 in them, so these two effects diminish with the square of the distance. The first term is just Coulomb’s Law (note that the minus sign in front takes care of the fact that like charges repel and so the E vector will point in the other way). Well… It is and it isn’t, because of the retarded time argument, of course. And so we have the second term, which sort of compensates for that. Indeed, the d(er)/dt is the time rate of change of er and, hence, if r‘/c = Δt, then (r‘/cd(er)/dt is a first-order approximation of Δer.

As Feynman puts it: “The second term is as though nature were trying to allow for the fact that the Coulomb effect is retarded, if we might put it very crudely. It suggests that we should calculate the delayed Coulomb field but add a correction to it, which is its rate of change times the time delay that we use. Nature seems to be attempting to guess what the field at the present time is going to be, by taking the rate of change and multiplying by the time that is delayed.” In short, the first two terms can be written as E = −(q/4πε0)/r2·[er + Δer] and, hence, it’s a sort of modified Coulomb Law that sort of tries to guess what the electrostatic field at P should be based on (a) what it is right now, and (b) how q’s direction and velocity, as measured now, would change it.

Now, the third term has a 1/c2 factor in front but, unlike the other two terms, this effect does not fall off with distance. So the formula below fully describes electromagnetic radiation, indeed, because it’s the only important term when we get ‘far enough away’, with ‘far enough’ meaning that the parts that go as the square of the distance have fallen off so much that they’re no longer significant.

Of course, you’re smart, and so you’ll immediately note that, as r increases, that unit vector keeps wiggling but that effect will also diminish. You’re right. It does, but in a fairly complicated way. The acceleration of er has two components indeed. One is the transverse or tangential piece, because the end of er goes up and down, and the other is a radial piece because it stays on a sphere and so it changes direction. The radial piece is the smallest bit, and actually also varies as the inverse square of r when r is fairly large. The tangential piece, however, varies only inversely as the distance, so as 1/r. So, yes, the wigglings of er look smaller and smaller, inversely as the distance, but the tangential piece is and remains significant, because it does not vary as 1/r2 but as 1/r only.  That’s why you’ll usually see the law of radiation written in an even simpler way:

This law reduces the whole effect to the component of the acceleration that is perpendicular to the line of sight only. It assumes the distance is huge as compared to the distance over which the charge is moving and, therefore, that r‘ and r can be equated for all practical purposes. It also notes that the tangential piece is all that matters, and so it equates d2(er)/dtwith ax/r. The whole thing is probably best illustrated as below: we have a generator driving charges up and down in G – so it’s an antenna really – and so we’ll measure a strong signal when putting the radiation detector D in position 1, but we’ll measure nothing in position 3. [The detector is, of course, another antenna, but with an amplifier for the signal.] But so here I am starting to talk about electromagnetic radiation once more, which was not what I wanted to do here, if only because Feynman does a much better job at that than I could ever do. 🙂

# Refraction and Dispersion of Light

In this post, we go right at the heart of classical physics. It’s going to be a very long post – and a very difficult one – but it will really give you a good ‘feel’ of what classical physics is all about. To understand classical physics – in order to compare it, later, with quantum mechanics – it’s essential, indeed, to try to follow the math in order to get a good feel for what ‘fields’ and ‘charges’ and ‘atomic oscillators’ actually represent.

As for the topic of this post itself, we’re going to look at refraction again: light gets dispersed as it travels from one medium to another, as illustrated below.

Dispersion literally means “distribution over a wide area”, and so that’s what happens as the light travels through the prism: the various frequencies (i.e. the various colors that make up natural ‘white’ light) are being separated out over slightly different angles. In physics jargon, we say that the index of refraction depends on the frequency of the wave – but so we could also say that the breaking angle depends on the color. But that sounds less scientific, of course. In any case, it’s good to get the terminology right. Generally speaking, the term refraction (as opposed to dispersion) is used to refer to the bending (or ‘breaking’) of light of a specific frequency only, i.e. monochromatic light, as shown in the photograph below. […] OK. We’re all set now.

It is interesting to note that the photograph above shows how the monochromatic light is actually being obtained: if you look carefully, you’ll see two secondary beams on the left-hand side (with an intensity that is much less than the central beam – barely visible in fact). That suggests that the original light source was sent through a diffraction grating designed to filter only one frequency out of the original light beam. That beam is then sent through a bloc of transparent material (plastic in this case) and comes out again, but displaced parallel to itself. So the block of plastics ‘offsets’ the beam. So how do we explain that in classical physics?

The index of refraction and the dispersion equation

As I mentioned in my previous post, the Greeks had already found out, experimentally, what the index of refraction was. To be more precise, they had measured the θ1 and θ2 – depicted below – for light going from air to water. For example, if the angle in air (θ1) is 20°, then the angle in the water (θ2) will be 15°. It the angle in air is 70°, then the angle in the water will be 45°.

Of course, it should be noted that a lot of the light will also be reflected from the water surface (yes, imagine the romance of the image of the moon reflected on the surface of glacial lake while you’re feeling damn cold) – but so that’s a phenomenon which is better  explained by introducing probability amplitudes, and looking at light as a bundle of photons, which we will not do here. I did that in previous posts, and so here, we will just acknowledge that there is a reflected beam but not say anything about it.

In any case, we should go step by step, and I am not doing that right now. Let’s first define the index of refraction. It is a number n which relates the angles above through the following relationship, which is referred to as Snell’s Law:

sinθ1 = n sinθ2

Using the numbers given above, we get: sin(20°) = n sin(15°), and sin(70°) = n sin(45°), so n must be equal to n = sin(20°)/sin(15°)  = sin(70°)/sin(45°) ≈ 1.33. Just for the record, Willibrord Snell was a medieval Dutch astronomer but, according to Wikipedia, some smart Persian, Ibn Sahl, had already jotted this down in a treatise – “On Burning Mirrors and Lenses” – while he was serving the Abbasid court of Baghdad, back in 984, i.e. more than a thousand years ago! What to say? It was obviously a time when the Sunni-Shia divide did not matter, and Arabs and ‘Persians’ were leading civilization. I guess I should just salute the Islamic Golden Age here, regret the time lost during Europe’s Dark Ages and, most importantly, regret where Baghdad is right now ! And, as for the ‘burning’ adjective, it just refers to the fact that large convex lenses can concentrate the sun’s rays to a very small area indeed, thereby causing ignition. [It seems that story about Archimedes burning Roman ships with a ‘death ray’ using mirrors – in all likelihood: something that did not happen – fascinated them as well.]

But let’s get back at it. Where were we? Oh – yes – the refraction index. It’s (usually) a positive number written as n = 1 + some other number which may be positive or negative, and which depends on the properties of the material. To be more specific, it depends on the resonant frequencies of the atoms (or, to be precise, I should say: the resonant frequencies of the electrons bound by the atom, because it’s the charges that generate the radiation). Plus a whole bunch of natural constants that we have encountered already, most of which are related to electrons. Let me jot down the formula – and please don’t be scared away now (you can stop a bit later, but not now 🙂 please):

N is just the number of charges (electrons) per unit volume of the material (e.g. the water, or that block of plastic), and qe and m are just the charge and mass of the electron. And then you have that electric constant once again, ε0, and… Well, that’s it ! That’s not too terrible, is it? So the only variables on the right-hand side are ω0 and ω, so that’s (i) the resonant frequency of the material (or the atoms – well, the electrons bound to the nucleus, to be precise, but then you know what I mean and so I hope you’ll allow me to use somewhat less precise language from time to time) and (ii) the frequency of the incoming light.

The equation above is referred to as the dispersion relation. It’s easy to see why: it relates the frequency of the incoming light to the index of refraction which, in turn, determinates that angle θ. So the formula does indeed determine how light gets dispersed, as a function of the frequencies in it, by some medium indeed (glass, air, water,…).

So the objective of this post is to show how we can derive that dispersion relation using classical physics only. As usual, I’ll follow Feynman – arguably the best physics teacher ever. 🙂 Let me warn you though: it is not a simple thing to do. However, as mentioned above, it goes to the heart of the “classical world view” in physics and so I do think it’s worth the trouble. Before we get going, however, let’s look at the properties of that formula above, and relate it some experimental facts, in order to make sure we more or less understand what it is that we are trying to understand. 🙂

First, we should note that the index of refraction has nothing to do with transparency. In fact, throughout this post, we’ll assume that we’re looking at very transparent materials only, i.e. materials that do not absorb the electromagnetic radiation that tries to go through them, or only absorb it a tiny little bit. In reality, we will have, of course, some – or, in the case of opaque (i.e. non-transparent) materials, a lot – of absorption going on, but so we will deal with that later. So, let me repeat: the index of refraction has nothing to do with transparency. A material can have a (very) high index of refraction but be fully transparent. In fact, diamond is a case in point: it has one of the highest indexes of refraction (2.42) of any material that’s naturally available, but it’s – obviously – perfectly transparent. [In case you’re interested in jewellery, the refraction index of its most popular substitute, cubic zirconia, comes very close (2.15-2.18) and, moreover, zirconia actually works better as a prism, so its disperses light better than diamond, which is why it reflects more colors. Hence, real diamond actually sparkles less than zirconia! So don’t be fooled! :-)]

Second, it’s obvious that the index of refraction depends on two variables indeed: the natural, or resonant frequency, ω0, and the frequency ω, which is the frequency of the incoming light. For most of the ordinary gases, including those that make up air (i.e. nitrogen (78%) and oxygen (21%), plus some vapor (averaging 1%) and the so-called noble gas argon (0.93%) – noble because, just like helium and neon, it’s colorless, odorless and doesn’t react easily), the natural frequencies of the electron oscillators are close to the frequency of ultraviolet light. [The greenhouse gases are a different story – which is why we’re in trouble on this planet. Anyway…] So that’s why air absorbs most of the UV, especially the cancer-causing ultraviolet-C light (UVC), which is formally classified as a carcinogen by the World Health Organization. The wavelength of UVC light is 100 to 300 nanometer – as opposed to visible light, which has a wavelength ranging from 400 to 700 nm – and, hence, the frequency of UV light is in the 1000 to 3000 Teraherz range (1 THz = 1012 oscillations per second) – as opposed to visible light, which has a frequency in the range of 400 to 800 THz. So, because we’re squaring those frequencies in the formula, ω2 can then be disregarded in comparison with ω02: for example, 15002 = 2,250,000 and that’s not very different from 15002 – 5002 = 2,000,000. Hence, if we leave the ω2 out, we are still dividing by a very large number. That’s why n is very close to one for visible light entering the atmosphere from space (i.e. the vacuum). Its value is, in fact, around 1.000292 for incoming light with a wavelength of 589.3 nm (the odd value is the mean of so-called sodium D light, a pretty common yellow-orange light (street lights!), so that’s why it’s used as a reference value – however, don’t worry about it).

That being said, while the n of air is close to one for all visible light, the index is still slightly higher for blue light as compared to red light, and that’s why the sky is blue, except in the morning and evening, when it’s reddish. Indeed, the illustration below is a bit silly, but it gives you the idea. [I took this from http://mathdept.ucr.edu/ so I’ll refer you to that for the full narrative on that. :-)]

Where are we in this story? Oh… Yes. Two frequencies. So we should also note that – because we have two frequency variables – it also makes sense to talk about, for instance, the index of refraction of graphite (i.e. carbon in its most natural occurrence, like in coal) for x-rays. Indeed, coal is definitely not transparent to visible light (that has to do with the absorption phenomenon, which we’ll discuss later) but it is very ‘transparent’ to x-rays. Hence, we can talk about how graphite bends x-rays, for example. In fact, the frequency of x-rays is much higher than the natural frequency of the carbon atoms and, hence, in this case we can neglect the w02 factor, so we get a denominator that is negative (because only the -w2 remains relevant), so we get a refraction index that is (a bit) smaller than 1. [Of course, our body is transparent to x-rays too – to a large extent – but in different degrees, and that’s why we can take x-ray photographs of, for example, a broken rib or leg.]

OK. […] So that’s just to note that we can have a refraction index that is smaller than one and that’s not ‘anomalous’ – even if that’s a historical term that has survived.

Finally, last but not least as they say, you may have heard that scientists and engineers have managed to construct so-called negative index metamaterials. That matter is (much) more complicated than you might think, however, and so I’ll refer you to the Web if you want to find out more about that.

Light going through a glass plate: the classical idea

OK. We’re now ready to crack the nut. We’ll closely follow my ‘Great Teacher’ Feynman (Lectures, Vol. I-31) as he derives that formula above. Let me warn you again: the narrative below is quite complicated, but really worth the trouble – I think. The key to it all is the illustration below. The idea is that we have some electromagnetic radiation emanating from a far-away source hitting a glass plate – or whatever other transparent material. [Of course, nothing is to scale here: it’s just to make sure you get the theoretical set-up.]

So, as I explained in my previous post, the source creates an oscillating electromagnetic field which will shake the electrons up and down in the glass plate, and then these shaking electrons will generate their own waves. So we look at the glass as an assembly of little “optical-frequency radio stations” indeed, that are all driven with a given phase. It creates two new waves: one reflecting back, and one modifying the original field.

Let’s be more precise. What do we have here? First, we have the field that’s generated by the source, which is denoted by Es above. Then we have the “reflected” wave (or field – not much difference in practice), so that’s Eb. As mentioned above, this is the classical theory, not the quantum-electrodynamical one, so we won’t say anything about this reflection really: just note that the classical theory acknowledges that some of the light is effectively being reflected.

OK. Now we go to the other side of the glass. What do we expect to see there? If we would not have the glass plate in-between, we’d have the same Es field obviously, but so we don’t: there is a glass plate. 🙂 Hence, the “transmitted” wave, or the field that’s arriving at point P let’s say, will be different than Es. Feynman writes it as Es + Ea

Hmm… OK. So what can we say about that? Not easy…

The index of refraction and the apparent speed of light in a medium

Snell’s Law – or Ibn Sahl’s Law – was re-formulated, by a 17th century French lawyer with an interesting in math and physics, Pierre de Fermat, as the Principle of Least Time. It is a way of looking at things really – but it’s very confusing actually. Fermat assumed that light traveling through a medium (water or glass, for instance) would travel slower, by a certain factor n, which – indeed – turns out to be the index of refraction. But let’s not run before we can walk. The Principle is illustrated below. If light has to travel from point S (the source) to point D (the detector), then the fastest way is not the straight line from S to D, but the broken S-L-D line. Now, I won’t go into the geometry of this but, with a bit of trial and error, you can verify for yourself that it turns out that the factor n will indeed be the same factor n as the one which was ‘discovered’ by Ibn Sahl: sinθ1 = n sinθ2.

What we have then, is that the apparent speed of the wave in the glass plate that we’re considering here will be equal to v = c/n. The apparent speed? So does that mean it is not the real speed? Hmm… That’s actually the crux of the matter. The answer is: yes and no. What? An ambiguous answer in physics? Yes. It’s ambiguous indeed. What’s the speed of a wave? We mentioned above that n could be smaller than one. Hence, in that case, we’d have a wave traveling faster than the speed of light. How can we make sense of that?

We can make sense of that by noting that the wave crests or nodes may be traveling faster than c, but that the wave itself – as a signal – cannot travel faster than light. It’s related to what we said about the difference between the group and phase velocity of a wave. The phase velocity – i.e. the nodes, which are mathematical points only – can travel faster than light, but the signal as such, i.e. the wave envelope in the illustration below, cannot.

What is happening really is the following. A wave will hit one of these electron oscillators and start a so-called transient, i.e. a temporary response preceding the ‘steady state’ solution (which is not steady but dynamic – confusing language once again – so sorry!). So the transient settles down after a while and then we have an equilibrium (or steady state) oscillation which is likely to be out of phase with the driving field. That’s because there is damping: the electron oscillators resist before they go along with the driving force (and they continue to put up resistance, so the oscillation will die out when the driving force stops!). The illustration below shows how it works for the various cases:

In case (b), the phase of the transmitted wave will appear to be delayed, which results in the wave appearing to travel slower, because the distance between the wave crests, i.e. the wavelength λ, is being shortened. In case (c), it’s the other way around: the phase appears to be advanced, which translated into a bigger distance between wave crests, or a lengthening of the wavelength, which translates into an apparent higher speed of the transmitted wave.

So here we just have a mathematical relationship between the (apparent) speed of a wave and its wavelength. The wavelength is the (apparent) speed of the wave (that’s the speed with which the nodes of the wave travel through space, or the phase velocity) divided by the frequency: λ = vp/f. However, from the illustration above, it is obvious that the signal, i.e. the start of the wave, is not earlier – or later – for either wave (b) and (c). In fact, the start of the wave, in time, is exactly the same for all three cases. Hence, the electromagnetic signal travels at the same speed c, always.

While this may seem obvious, it’s quite confusing, and therefore I’ll insert one more illustration below. What happens when the various wave fronts of the traveling field hit the glass plate (coming from the top-left hand corner), let’s say at time t = t0, as shown below, is that the wave crests will have the same spacing along the surface. That’s obvious because we have a regular wave with a fixed frequency and, hence, a fixed wavelength λ0, here. Now, these wave crests must also travel together as the wave continues its journey through the glass, which is what is shown by the red and green arrows below: they indicate where the wave crest is after one and two periods (T and 2T) respectively.

To understand what’s going on, you should note that the frequency f of the wave that is going through the glass sheet and, hence, its period T, has not changed. Indeed, the driven oscillation, which was illustrated for the two possible cases above (n > 1 and n < 1), after the transient has settled down, has the same frequency (f) as the driving source. It must. Always. That being said, the driven oscillation does have that phase delay (remember: we’re in the (b) case here, but we can make a similar analysis for the (c) case). In practice, that means that the (shortest) distance between the crests of the wave fronts at time t = t0 and the crests at time t0 + T will be smaller. Now, the (shortest) distance between the crests of a wave is, obviously, the wavelength divided by the frequency: λ = vp/f, with vp the speed of propagation, i.e. the phase velocity, of the wave, and f = 1/T. [The frequency f is the reciprocal of the period T – always. When studying physics, I found out it’s useful to keep track of a few relationships that hold always, and so this is one of them. :-)]

Now, the frequency is the same, but so the wavelength is shortened as the wave travels through the various layers of electron oscillators, each causing a delay of phase – and, hence, a shortening of the wavelength, as shown above. But, if f is the same, and the wavelength is shorter, then vp cannot be equal to the speed of the incoming light, so vp ≠ c. The apparent speed of the wave traveling through the glass, and the associated shortening of the wavelength, can be calculated using Snell’s Law. Indeed, knowing that n ≈ 1.33, we can calculate the apparent speed of light through the glass as v = c/n  ≈ 0.75c and, therefore, we can calculate the wavelength of the wave in the glass l as λ = 0.75λ0.

OK. I’ve been way too lengthy here. Let’s sum it all up:

• The field in the glass sheet must have the shape that’s depicted above: there is no other way. So that means the direction of ‘propagation’ has been changed. As mentioned above, however, the direction of propagation is a ‘mathematical’ property of the field: it’s not the speed of the ‘signal’.
• Because the direction of propagation is normal to the wave front, it implies that the bending of light rays comes about because the effective speed of the waves is different in the various materials or, to be even more precise, because the electron oscillators cause a delay of phase.
• While the speed and direction of propagation of the wave, i.e. the phase velocity, accurately describes the behavior of the field, it is not the speed with which the signal is traveling (see above). That is why it can be larger or smaller than c, and so it should not raise any eyebrow. For x-rays in particular, we have a refractive index smaller than one. [It’s only slightly less than one, though, and, hence, x-ray images still have a very good resolution. So don’t worry about your doctor getting a bad image of your broken leg. 🙂 In case you want to know more about this: just Google x-ray optics, and you’ll find loads of information. :-)]

Calculating the field

Are you still there? Probably not. If you are, I am afraid you won’t be there ten or twenty minutes from now. Indeed, you ain’t done nothing yet. All of the above was just setting the stage: we’re now ready for the pièce de résistance, as they say in French. We’re back at that illustration of the glass plate and the various fields in front and behind the plate. So we have electron oscillators in the glass plate. Indeed, as Feynman notes: “As far as problems involving light are concerned, the electrons behave as though they were held by springs. So we shall suppose that the electrons have a linear restoring force which, together with their mass m, makes them behave like little oscillators, with a resonant frequency ω0.”

So here we go:

1. From everything I wrote about oscillators in previous posts, you should remember that the equation for this motion can be written as m[d2x/dt2 + ω02) = F. That’s just Newton’s Law. Now, the driving force F comes from the electric field and will be equal to F = qeEs.

Now, we assume that we can chose the origin of time (i.e. the moment from which we start counting) such that the field Es = E0cos(ωt). To make calculations easier, we look at this as the real part of a complex function Es = E0eiωt. So we get:

m[d2x/dt2 + ω02] = qeE0eiωt

We’ve solved this before: its solution is x = x0eiωt. We can just substitute this in the equation above to find x0 (just substitute and take the first- and then second-order derivative of x indeed): x0 = qeE0/m(ω022). That, then, gives us the first piece in this lengthy derivation:

x = qeE0eiωt/m(ω02 2)

Just to make sure you understand what we’re doing: this piece gives us the motion of the electrons in the plate. That’s all.

2. Now, we need an equation for the field produced by a plane of oscillating charges, because that’s what we’ve got here: a plate or a plane of oscillating charges. That’s a complicated derivation in its own, which I won’t do there. I’ll just refer to another chapter of Feynman’s Lectures (Vol. I-30-7) and give you the solution for it (if I wouldn’t do that, this post would be even longer than it already is):

This formula introduces just one new variable, η, which is the number of charges per unit area of the plate (as opposed to N, which was the number of charges per unit volume in the plate), so that’s quite straightforward. Less straightforward is the formula itself: this formula says that the magnitude of the field is proportional to the velocity of the charges at time t – z/c, with z the shortest distance from P to the plane of charges. That’s a bit odd, actually, but so that’s the way it comes out: “a rather simple formula”, as Feynman puts it.

In any case, let’s use it. Differentiating x to get the velocity of the charges, and plugging it into the formula above yields:

Note that this is only Ea, the additional field generated by the oscillating charges in the glass plate. To get the total electric field at P, we still have to add Es, i.e. the field generated by the source itself. This may seem odd, because you may think that the glass plate sort of ‘shields’ the original field but, no, as Feynman puts it: “The total electric field in any physical circumstance is the sum of the fields from all the charges in the universe.”

3. As mentioned above, z is the distance from P to the plate. Let’s look at the set-up here once again. The transmitted wave, or Eafter the plate as we shall note it, consists of two components: Es and Ea. Es here will be equal to (the real part of) Es = E0eiω(t-z/c). Why t – z/c instead of just t? Well… We’re looking at Es here as measured in P, not at Es at the glass plate itself.

Now, we know that the wave ‘travels slower’ through the glass plate (in the sense that its phase velocity is less, as should be clear from the rather lengthy explanation on phase delay above, or – if n would be greater than one – a phase advance). So if the glass plate is of thickness Δz, and the phase velocity is is v = c/n, then the time it will take to travel through the glass plate will be Δz/(c/n) instead of Δz/c (speed is distance divided by time and, hence, time = distance divided by speed). So the additional time that is needed is Δt = Δz/(c/n) – Δz/c = nΔz/c – Δz/c = (n-1)Δz/c. That, then, implies that Eafter the plate is equal to a rather monstrously looking expression:

Eafter plate = E0eiω[t (n1)Δz/c z/c) = eiω(n1)Δz/c)E0eiω(t z/c)

We get this by just substituting t for t – Δt.

So what? Well… We have a product of two complex numbers here and so we know that this involves adding angles – or substracting angles in this case, rather, because we’ve got a minus sign in the exponent of the first factor. So, all that we are saying here is that the insertion of the glass plate retards the phase of the field with an amount equal to w(n-1)Δz/c. What about that sum Eafter the plate = Es + Ea that we were supposed to get?

Well… We’ll use the formula for a first-order (linear) approximation of an exponential once again: ex ≈ 1 + x. Yes. We can do that because Δz is assumed to be very small, infinitesimally small in fact. [If it is not, then we’ll just have to assume that the plate consists of a lot of very thin plates.] So we can write that eiω(n-1)Δz/c) = 1 – iω(n-1)Δz/c, and then we, finally, get that sum we wanted:

Eafter plate = E0eiω[t z/c) iω(n-1)Δz·E0eiω(t z/c)/c

The first term is the original Es field, and the second term is the Ea field. Geometrically, they can be represented as follows:

Why is Ea perpendicular to Es? Well… Look at the –i = 1/i factor. Multiplication with –i amounts to a clockwise rotation by 90°, and then just note that the magnitude of the vector must be small because of the ω(n-1)Δz/c factor.

4. By now, you’ve either stopped reading (most probably) or, else, you wonder what I am getting at. Well… We have two formulas for Ea now:

and Ea = – iω(n-1)Δz·E0eiω(t – z/c)/c

Equating both yields:

But η, the number of charges per unit area, must be equal to NΔz, with N the number of charges per unit volume. Substituting and then cancelling the Δz finally gives us the formula we wanted, and that’s the classical dispersion relation whose properties we explored above:

Absorption and the absorption index

The model we used to explain the index of refraction had electron oscillators at its center. In the analysis we did, we did not introduce any damping factor. That’s obviously not correct: it means that a glass plate, once it had illuminated, would continue to emit radiation, because the electrons would oscillate forever. When introducing damping, the denominator in our dispersion relation becomes m(ω02 – ω2 + iγω), instead of m(ω02 – ω2). We derived this in our posts on oscillators. What it means is that the oscillator continues to oscillate with the same frequency as the driving force (i.e. not its natural frequency) – so that doesn’t change – but that there is an envelope curve, ensuring the oscillation dies out when the driving force is no longer being applied. The γ factor is the damping factor and, hence, determines how fast the damping happens.

We can see what it means by writing the complex index of refraction as n = n’ – in’’, with n’ and n’’ real numbers, describing the real and imaginary part of n respectively. Putting that complex n in the equation for the electric field behind the plate yields:

Eafter plate = eωn’’Δz/ceiω(n’1)Δz/cE0eiω(t z/c)

This is the same formula that we had derived already, but so we have an extra exponential factor: eωn’’Δz/c. It’s an exponential factor with a real exponent, because there were two i‘s that cancelled. The e-x function has a familiar shape (see below): e-x is 1 for x = 0, and between 0 and 1 for any value in-between. That value will depend on the thickness of the glass sheet. Hence, it is obvious that the glass sheet weakens the wave as it travels through it. Hence, the wave must also come out with less energy (the energy being proportional to the square of the amplitude). That’s no surprise: the damping we put in for the electron oscillators is a friction force and, hence, must cause a loss of energy.

Note that it is the n’’ term – i.e. the imaginary part of the refractive index n – that determines the degree of absorption (or attenuation, if you want). Hence, n’’ is usually referred to as the “absorption index”.

The complete dispersion relation

We need to add one more thing in order to get a fully complete dispersion relation. It’s the last thing: then we have a formula which can really be used to describe real-life phenomena. The one thing we need to add is that atoms have several resonant frequencies – even an atom with only one electron, like hydrogen ! In addition, we’ll usually want to take into account the fact that a ‘material’ actually consists of various chemical substances, so that’s another reason to consider more than one resonant frequency. The formula is easily derived from our first formula (see the previous post), when we assumed there was only one resonant frequency. Indeed, when we have Nk electrons per unit of volume, whose natural frequency is ωk and whose damping factor is γk, then we can just add the contributions of all oscillators and write:

The index described by this formula yields the following curve:

So we have a curve with a positive slope, and a value n > 1, for most frequencies, except for a very small range of ω’s for which the slope is negative, and for which the index of refraction has a value n < 1. As Feynman notes, these ω’s– and the negative slope – is sometimes referred to as ‘anomalous’ dispersion but, in fact, there’s nothing ‘abnormal’ about it.

The interesting thing is the iγkω term in the denominator, i.e. the imaginary component of the index, and how that compares with the (real) “resonance term” ωk2– ω2. If the resonance term becomes very small compared to iγkω, then the index will become almost completely imaginary, which means that the absorption effect becomes dominant. We can see that effect in the spectrum of light that we receive from the sun: there are ‘dark lines’, i.e. frequencies that have been strongly absorbed at the resonant frequencies of the atoms in the Sun and its ‘atmosphere’, and that allows us to actually tell what the Sun’s ‘atmosphere’ (or that of other stars) actually consists of.

So… There we are. I am aware of the fact that this has been the longest post of all I’ve written. I apologize. But so it’s quite complete now. The only piece that’s missing is something on energy and, perhaps, some more detail on these electron oscillators. But I don’t think that’s so essential. It’s time to move on to another topic, I think.

Introduction: Scale Matters

One of the points which Richard Feynman, as a great physics teacher, does admirably well is to point out why scale matters. In fact, ‘old’ physics are not incorrect per se. It’s just that ‘new’ physics analyzes stuff at a much smaller scale.

For example, Snell’s Law, or Fermat’s Principle of Least Time, which were ‘discovered’ 500 years ago – and they are actually older, because they formalize something that the Greeks had already found out: refraction of light, as it travels from one medium (air, for example) into another (water, for example) – are still fine when studying focusing lenses and mirrors, i.e. geometrical optics. The dimensions of the analysis, or the equipment involved (i.e. the lenses or the mirrors), are huge as compared to the wavelength of the light and, hence, we can effectively look at light as a beam that travels from one point to another in a straight line, that bounces of a surface, or as a beam that gets refracted when it passes from one medium to another.

However, when we let the light pass through very narrow slits, it starts behaving like a wave. Geometrical optics does not help us, then, to understand its behavior: we will, effectively, analyze light as a wave-like thing at that scale, and analyze wave-like phenomena, such as interference, the Doppler effect and what have you. That level of analysis is referred to as the classical theory of electromagnetic radiation, and it’s what we’ll be introducing in this post.

The analysis of light as photons, i.e. as a bunch of ‘particles’ described by some kind of ‘wave function’ (which does not describe any real wave, but only some ‘probability amplitude’), is the third and final level of analysis, referred to as quantum mechanics or, to be more precise, as quantum electrodynamics (QED). [Note the terminology: quantum mechanics describes the behavior of matter particles, such as protons and electrons, while quantum electrodynamics (QED) describes the nature of photons, a force-carrying particle, and their interaction with matter particles.]

But so we’ll focus on the second level of analysis in this post.

Different mathematical approaches

One other thing which Feynman points out in his Lectures is that, even within a well-agreed level of analysis, there are different mathematical approaches to a problem. In fact, while, at any level of analysis, there’s (probably) only one fully mathematically correct analysis, approximate approaches may actually be easier to work with, not only because they actually allow us to solve a practical problem, but also because they help us to understand what’s going on.

Feynman’s treatment of electromagnetic radiation (Volume I, Chapters 28 to 34) is a case in point. While he notes that Maxwell’s field equations are actually the ones to be used, he writes them in a mathematical form that we can understand more easily, and then simplifies that mathematical form even further, in order to derive all that a sophomore student is supposed to know about electromagnetic radiation (EMR), which, of course, not only includes what we call light but also radio waves, radar waves, infrared waves and, on the other side of the spectrum, x-rays and gamma rays.

But let’s get down to business now.

The oscillating charge

Radiation is caused by some far-away electric charge (q) that’s moving in various directions in a non-uniform way, i.e. it is accelerating or decelerating, and perhaps reversing direction in the process. From our point of view (P), we draw a unit vector er’ in the direction of the charge. [If you want a drawing, there’s one further down.]

We write r’ (r prime), not r, because it is the retarded distance: when we look at the charge, we see where it was r’/c seconds ago: r’/c is indeed the time that’s needed for some influence to travel from the charge to the here and now, i.e. to P. So now we can write Coulomb’s Law:

E1 = –qer’/4πe0r’2

This formula can quickly be explained as follows:

1. The minus sign makes the direction of the force come out alright: like charges do not attract but repel, unlike gravitation. [Indeed, for gravitation, there’s only one ‘charge’, a mass, and masses always attract. Hence, for gravitation, the force law is that like charges attract, but so that’s not the case here.]
2. E and er’ and, hence, the electric force, are all directed along the line of sight.
3. The Coulomb force is proportional to the amount of charge, and the factor of proportionality is 1/4πe0r’2.
4. Finally, and most importantly in this context (study of EMR), the influence quickly diminishes with the distance: it varies inversely as the square of the distance (i.e. it varies as the inverse square).

Coulomb’s Law is not all that comes out of Maxwell’s field equations. Maxwell’s equations also cover electrodynamics. Fortunately, because we are, indeed, talking moving charges here, so electrostatics is only part of the picture and, in fact, the least important one in this case. 🙂 That’s why I wrote E1, with as subscript, above – not E.

So we have a second term, and I’ll actually be introducing a third term in a minute or so. But let’s first look at the second term. I am not sure how Feynman derives it from Maxwell’s equations – I am sure I’ll see the light 🙂 when reading Volume II – but, from Maxwell’s equations, he does, somehow, derive the following, secondary, effect:

This is a term I struggled with in a first read, and I still do. As mentioned above, I need to read Feynman’s Volume II, I guess. But, while I still don’t understand the why, I now understand what this expression catches. The term between brackets is the Coulomb effect, which we mentioned above already, and the time derivative is the rate of change. We multiply that with the time delay (i.e. r’/c). So what’s going on? As Feynman writes it: “Nature seems to be attempting to guess what the field at the present time is going to be, by taking the rate of change and multiplying by the time that is delayed.”

OK. As said, I don’t really understand where this formula comes from but it makes sense, somehow. As for now, we just need to answer another question in order to understand what’s going on: in what direction is the Coulomb field changing?

It could be either: if the charge is moving along the direction of sight er’ won’t change but r’ will. However, if r’ does not change, then it’s er’ that changes direction, and that change will be perpendicular to the line of sight, or transverse (as opposed to radial), as Feynman puts it. Or, of course, it could be a combination of both. [Don’t worry too much if you’re not getting this: we will need this again in just a minute or so, and then I will also give you a drawing so you’ll see what I mean.]

The point is, these first two terms are actually not important because electromagnetic radiation is given by the third effect, which is written as:

Wow ! This looks even more complicated, doesn’t it? Let’s analyze it. The first thing to note is that there is no r’ or r’2 in this equation. However, that’s an optical illusion of sorts, because r’ does matter when looking at that second-order derivative. How? Well… Let’s go step by step and first look at that second-order derivative. It’s the acceleration (or deceleration) of er’. Indeed, visualize er’ wiggling about, trying to follow the charge by pointing at where the charge was r’/c seconds ago. Let me help you here by, finally, inserting hat drawing I promised you.

This acceleration will have a transverse as well as a radial component: we can imagine the end of er’ (i.e. the point of the arrow) being on the surface of a unit sphere indeed. So as it wiggles about, the tip of the arrow moves back a bit from the tangential line. That’s the radial component of the acceleration. It’s easy to see that it’s quite small as compared to the transverse component, which is the component along the line that’s tangent to the surface (i.e. perpendicular to er’).

Now, we need to watch out: we are not talking displacement or velocity here but acceleration. Hence, even if the displacement of the charge is very small, and even if velocities would not be phenomenal either (i.e. non-relativistic), the acceleration involved can take on any value really. Hence, even with small displacements, we can have large accelerations, so the radial component is small relative to the transverse component only, not in an absolute sense.

That being said, it’s easy to see that both the transverse as well as the radial component depend on the distance r’ but in a different way. I won’t bother you with the geometrical proof (it’s not that obvious). Just accept that the radial component varies, more or less as the inverse square of the distance. Hence, we will simplify and say that we’re considering large distances r’ only – i.e. large in comparison to the length of the unit vector, which just means large in comparison to one (1) – and then it’s only the transverse component of a that matters, which we’ll denote by ax.

However, if we drop that radial component, then we should drop E1 as well, because the Coulomb effect will be very small as compared to the radiation effect (i.e. E3). And, then, if we drop E1, we can drop the ‘correction’ E2 as well, of course. Indeed, that’s what Feynman does. He ends up with this third term only, which he terms the law of radiation:

So there we are. That’s all I wanted to introduce here. But let’s analyze it a bit more. Just to make sure we’re all getting it here.

All that simplification business above is tricky, you’ll say. First, why do we write t – r/c for the retarded time (t’)? It should be t – r’/c, no? You’re right. There’s another simplification here: we fix the delay time, assuming that the charge only moves very small distances at an effectively constant distance r. Think of some far-away antenna indeed.

Hmm… But then we have that 1/c2 factor, so that should reduce the effect to zilch, isn’t it? And then… Hey! Wait a minute! Where does that r suddenly come from? Well, we’ve replaced d2er’/dt2 by the lateral acceleration of the charge itself (i.e. its component perpendicular to the line of sight, denoted by ax) divided by r. That’s just similar triangles.

Phew! That’s a lot of simplifications and/or approximations indeed. How do we know this law really works? And, if it does, for what distance? When is that 1/r part (i.e. E3) so large as compared to the other two terms (E1 and E2) that the latter two don’t matter anymore? Well… That seems to depend on the wavelength of the radiation, but we haven’t introduced that concept yet. Let me conclude this first introduction by just noting this ‘law’ can easily be confirmed by experiment.

A so-called dipole oscillator or radiator can be constructed, as shown below: a generator drives electrons up and down in two wires (A and B). Why do we put the generator in the middle? That’s because we want a net effect: the radiation effect of the electrons in the wires connecting the generator with A and B will be neutral, because the electrons move right next to each other in opposite direction. With the generator in the middle, A and B form one antenna, which we’ll denote by G (for generator).

Now, another antenna can act as a receiver, and we can amplify the signal to hear it. That’s the D (for detector) shown below. Now, one of the consequences of the above ‘law’ for electromagnetic radiation is, obviously, that the strength of the received signal should become weaker as we turn the detector. The strongest signal should be when D is parallel to G. At point 2, there is a projection effect and, hence, the strength of the field should be less. Indeed, remember that the strength of the field is proportional to the acceleration of the charge projected perpendicular to the line of sight. Hence, at point 3, it should be zero, because the projection is zero.

Now, that’s what an experiment like this would indeed confirm. [I am tempted now to explain how a radio receiver works, but I will resist the temptation.]

I just need to make a last point here in order to make sure that we understand the formula above and – more importantly – that we can use in subsequent chapters without having to wonder where it comes from. The formula above implies that the direction of the field is at right angles to the line of sight. Now, if a charge is just accelerating up and down, in a motion of very small amplitude, i.e. like the motion in that antenna, then the magnitude (or strength let’s say) of the field will be given by the following formula:

θ, in this formula, is the angle between the axis of motion and the line of sight, as illustrated below:

So… That’s all we need to know for now. We’re done. As for now that is. This was quite technical, I guess, but I am afraid the next post will be even more technical. Sorry for that. I guess this is just a piece we need to get through.

Post scriptum:

You’ll remember that, with moving and accelerating charges, we should also have a magnetic field, usually denoted by B. That’s correct. If we have a changing electric field, then we will also have a magnetic field. There’s a formula for B:

B = –er’´E/c = –| er’||E|c–1sin(er’, En = –(E/c)·n

This is a vector cross-product. The angle between the unit vector er’ and E is π/2, so the sine is one. The vector n is the vector normal to both vectors as defined by the right-hand screw rule. [As for the minus sign, note that –a´b = b´a, so we could have reversed the vectors: the minus sign just reverses the direction of the normal vector.] In short, the magnetic field vector B is perpendicular to E, but its magnitude is tiny: E/c. That’s why Feynman neglects it, but we will come back on that in later posts.