Classically, this can be defined in terms of self consistent electromagnetic fields. When matter is involved (conductors and dielectrics), charges move under the influence of the electric field but moving charges induce magnetic fields. These fields typically serve to cancel the imposed EM fields, "screening" at least a portion of the field. A perfect electric conductor completely screens the field, which is why electric field can only be perpendicular to the surface of a conductor since any field parallel to the surface moves electrons until it cancels itself out.

Dielectrics can't allow charges to move far, but they polarize under the field. In addition to altering the net field level, the induced oscillations of the charges can be thought of as a dipole or multipole source generating a new EM wave that attempts to cancel the applied wave. Since the induced wave radiates in all directions, at least a portion would propagate back along the wave, resulting in a net shift to the phase front of the wave and thus "slows" the wave within the dielectric. Feynman had a good lecture on this topic.

Hope that helps.

Michael

Michael D. Foegelle

It is not the radiation in the backward direction that delays the wave in a dielectric, but the phase-shifted re-radiation in the forward direction.

Dear Ketan Karkare :

I can remember two cases:

1. When Maxwell Equations are confined to waveguides (mostly for telecommunications)

2. Maxwell equations inside solids (metals and insulators/dielectrics), the theory was developed by Prof Lorentz and can be called "LORENTZ & MAXWELL" equations (Lorentz, 1902).

L. D. Landau, E. M. Lifshitz (1960). Electrodynamics of Continuous Media. Vol. 8 (1st ed.). Pergamon Press. ISBN 978-0-08-009105-1 gives an excellent formulation of the topic. First Chapter on the old book, the new one I haven't read.

Best Regards.

Aparna Sathya Murthy , at the risk of being branded a heretic, Maxwell's equations aren't really physics, they're a mathematical description of the physics. (Likewise Schrödinger's equation is a mathematical description of quantum mechanics.) It wasn't until I had a feel for the underlying mechanics of what was happening (e.g. charges moving under the influence of fields) that the rules of Maxwell's equations really made sense. Given that Ketan Karkare was asking about a non-mathematical description of the physics, that kind of precludes the use of Maxwell's equations as an explanation.

Malcolm White , from a purely classical perspective I generally agree with you, although I think it's simpler to think of the charges causing "drag" on the wave as the changing field attempts to accelerate the charges. The resulting field propagating through the material is the applied field plus the fields produced by the charges reacting to the applied field, which are always lagging the applied field.

However, as I recall Feynman's lecture on the subject, he was talking quantum mechanically and the topic had the usual deterministic issues associated with QM which he described as a wave (or perhaps a wave function) propagating backwards. It's been a while since I've read that, but that particular feature of the discussion stuck in my mind because it did seem counterintuitive.

Note that in the classical picture, the absorb and re-radiate picture (which is really a quantum mechanical concept) that gives rise to a reverse propagating wave would presumably create the reflected wave that we take as occurring at the boundary of the dielectric. However, that implies that surface reflectivity is a bulk property of the material and that the more material you had, the more would reflect. Perhaps that is the case but there's a limiting factor (akin to the skin depth) where after a certain depth, the charge to charge radiation interaction starts to screen itself so that the backwards radiation doesn't increase with the amount of material. That might explain thin film effects, but I don't quite see how it describes internal reflections at the boundary of a dielectric to free space. Unless it's the sudden end of the back radiated components causing a phase boundary that looks as though it comes from the surface. Can't say I've ever thought along these lines, but it might be interesting to play with. Essentially the (presumably periodic) structure of the dielectric becomes a phased array or better yet, a reflect-array that does funny things to the applied wave. After all, that's what metamaterials are!

Presumably this could be tested. If the reflection is purely a surface effect, then a transition (turning the signal off) would remain "sharp" when looking at a reflection. However, if it's an aggregate of internal re-radiation of the source then it would be blurred. Unfortunately I don't see any way to do this sort of analysis at radio frequencies as the wavelength and ring times of antennas, etc. would be too long. On the other hand, if a surface reflection had any depth associated with it, there would be a corresponding phase change that we don't see in reality, so... Or do we? The sum of small randomly out-of-phase waves has a much bigger impact on the amplitude of a wave than it does to the phase of the wave.

Oh well, enough rambling on ideas as I write this... Back to work!

Classical Maxwells equations are an excellent tool to describe with extremely high precision nearly all electromagnetic phenomena of confined and non confined EM fields; exceptions are e.g. extremely high fields in vacuum (photon-photon collider) and the so far hypothetical axion -EM coupling (modified Maxwelles equations fort his case.)However there are other modified Maxwells equations on the "market"which should be regarded with utmost caution.

The key evidence for light waves is the Young’s double-slit interference experiment. After the discovery of the double-slit interference experiment of particles (electrons, etc.), the conclusion of light waves can no longer be established. "Wave-particle duality" denies light as particles because of the wave phenomenon, but recognizes the wave nature of particles. It is better understood that light particles have wave phenomena. Of course, light particle behavior and wave mechanism need new understanding.

https://book4you.org/book/18269199/bf2351

The electric field of a charged particle has no force on itself, indicating that the electrostatic effect is the effect between the charged particles, not the effect between the field and the charged particle, and the electric field is an active field. Similarly, the magnetic field is also an active field. There are no flying passive electromagnetic fields (electromagnetic waves), light is a particle.

The behaviour of EM wave in free space or a confined region can be understood by understanding the basic concept of waves i.e. the Interference of Waves - Constructive and Destructive Interference.

Alternatively Far Field analysis of Antenna can also give an overview of the behaviour of EM wave.

The electric field of a charged particle has no force on itself, indicating that the electrostatic effect is the effect between the charged particles, not the effect between the field and the charged particle. It doesn't make sense to consider the electric field alone, ignoring the source of the field, the electric field is an active field. Similarly, the magnetic field is also an active field. There are no flying passive electromagnetic fields (electromagnetic waves), light is a particle.

Article Photon energy and photon behavior discussions