h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I know in general what matter is made of and that light is an alternating electric/magnetic field moving though space (although I don't entirely undrstand the latter). Electromagnetic radiation is affected by the varying electric and magnetic fields is matter, yes?

Which is more significant, electric or magnetic?

What causes light to slow down to varying degrees in different materials (refraction)? What causes light to reflect (like in mirrors)? Why doesn't glass affect visible light very much?

A bit deep this one and I'm no theoretical physicist but I'll have a go.

Matter is energy and energy is matter, a famous eqation E= mc^2 quantifies how much energy is innate in matter. Photons have zero mass so are effectively just an energy wave. At 90 degrees to the direction of propogation and perpendicular to one another are associated oscillating magnetic in electric fields as you say.

When atomic nuclei are around then electrons accumulate around them in discrete energy levels with finite and measurable energy gaps between them. The energy of a light wave is E = h x frequecy where E = energy and h is a constant (called Planck's constant). When a photon of exactly equal energy to an energy gap hits matter then it induces the electrons to leap between this energy gap. My guess is that nobody really *knows* the mechanism of this but it does happen - there is overwhelming evidence for it.

It is not just light that does this, all wavelenghts of electromagnetic radiation interact with different degrees of freedom for matter. For example microwaves interact with rotational energy levels of molecules. Infra-red interacts with molecular vibrations, X-rays and gamma rays interact with energy transisitons within atomic nuclei...

I think to explain refraction and diffraction of light then you need to go into general relativity. I think that a mass puts a 'bend' in space and that light bends with it as it essentially is travelling in a straight line but space itself is changed. Different masses cause differing amounts of this and hence light is affected according to the form of matter.

Glass won't interact with visible light because its chemical structure has no degrees of freedom (in this wavelength it would be energy gaps within the electronic structure) that equate to visible light wavelenghts and so they do not interact with it.

I don't think there is any significance to attach to magnetic or electric fields associated with the travelling light wave in this context as far as I know. The photon and the electrons affected have various properties of angular momentum, energy and other quantum properties all of which must be the same before and after the event. They just get transferred between species. For example and photon has no 'spin' and so there can be no change in electron spin when it is excited by a photon.

To your first question syme42, I would say no. The electromagnetic radiation isn't affected by the varying electric and magnetic fields in matter. The total electric and magnetic fields present are the sum of the electromagnetic radiation, and those fields present from the matter. And this is very different from the electromagnetic wave in the vacuum. But this doesn't change the electromagnetic radiation itself.

Generally speaking, to answer your question about radiation interacting with matter (especially in everyday life) the electric field is much more important than the magnetic.

Regarding light slowing down: from my textbook: maxwells equations that govern electromagnetic radiation are slighltly different when changing from vacuum to matter. The difference being that the electric permittivity and the magnetic susceptibility are changed.

What are those actually? Well, electric permittivity is how matter responds to the presence of an electric field. Non-conducting matter responds by becoming "polarized". That is, atoms and molecules within the matter reorient their charges/charged particles such that they line up with the applied electric field. Generally the alignment is far from perfect. How much the matter becomes polarized by an applied electric field is the "electric permittivity".

Magnetic susceptibility is the same thing, applied to magnetic fields. An applied magnetic field causes the microscopic magnetic dipoles within matter to tend to align with the field. The effectiveness of the applied magnetic field in causing the alignment is the "magnetic susceptibility".

The net effect of this polarization is to reduce the total field within the matter. So if an electric field of 1 volt/meter is applied to a piece of matter, within the matter the total field might only be 0.95 volts/meter. The same would be true for an applied magnetic field.

So this has told us that electric and magnetic field amplitudes are reduced in matter with respect to the vacuum. By doing the maths, and deriving the results for Maxwells equations, what we then learn is that the *rate of change of the fields* are also reduced in matter. So when the oscilating field enters a piece of matter, the amplitude is decreased, and it's rate of change also decreases. Since it is the change in electric field which leads to the speed of light, the speed of light is decreased in matter.

Imagine having a really tense spring attached end-to-end with a loose spring. If you push on the tense spring, it transmit the force very quickly to it's other end via a very fast moving wave. That in turn pushes on the loose spring - but this creates a slow moving wave. That is a loose analogy to explain why light slows down when it enters matter.


what orcus said about glass.


Mirrors - metals are conductors, and by definition electric and/or magnetic fields do not exist inside conductors. This is because the conducting electrons inside the conductor re-arrange themselves very,very quickly to cancel out any electric field (or begin flowing to cancel any magnetic field). When an electromagnetic field reaches the surface of a conductor, the electrons begin re-arranging and flowing to cancel the incoming fields. Their motion cancels the incoming field within the conductor, but also generate an outgoing field leaving the conducting. This outgoing field is the "reflected" version of the incoming one.

Ok, the explanation of refraction makes sense.

Mirrors - Cool, I forgot about that effect. A quick Google search turned up an explanation of why aluminum is more reflective than copper at visible frequencies: www-ssrl.slac.stanford.edu/lcls/technotes/LCLS-TN-05-6.pdf

Nice one Dealer, I can see I should shut up when it come to the physics stuff

thanks for the link syme42! I'll check that out.

Orcus don't stop! It's always a good starting point that gets me started. I have much harder time starting from scratch.

Don't worry, I find it very hard to shut up in reality

It's just that I keep saying, "nobody really knows..." and then you turn up and post not only why and how but it a really nice way that I can understand. I'll keep talking as long as you do

sounds like a good deal to me I try to explain it simply, but I know in the back of my mind that whatever I say, you're at least going to be able to make some sense of it.

I wonder if anyone else knows what we are talking about

well syme42 might have, but he seems to have some background. I'm not sure how the "casual" passerby would fair. We could as MoG to stroll through and take a look.

*passes by as casually as possible*

So are you saying that reflected light in a mirror isn't actually reflected at all, but rather a generated output by the material of the mirror in response to the incoming light?

yes!

that's quite, erm, radically different to what is commonly thought to happen isn't it?
A prime example of science showing that something that seems to be incredibly simple and obvious is actually wrong and a lot more complicated.
Interesting.

It's certainly more complex than what I was taught. I think that's because of the argument whether light is composed of particles or not.

It does seem to make sense. The incoming wave or particle is absorbed (maybe not the best word) but in doing so alters the energy levels of the substance, which re-equalises itself by emitting the same amount and type as it absorbed.

Thinking about it though, why does it come out in the same direction it went in?

I guess there is some sort of angular momentum thingy going on here which would explain the reflection 'effect' otherwise the output photon could come out at any angle and speed. Equally, this would explain why polished surfaces work better, since otherwise a very uneven surface could, due to probability, provide just as good a 'reflector' as a polished surface. so presumably the angle at which the photon hits the surface is also factored into the reaction somehow.

(actually, I'm still waiting for someone to post that this is a wind-up, it is so unlike what we mere mortals think happens!).

Imagine the metal having a uniform sea of electrons. Now an electromagnetic wave approaches it. The wave is polarized in a certain direction. The wave actually represents a force. It is a force which forces positive charge one way, negative charge the other. In the metal, the positive ion cores aren't going to be moving, but the electron sea is free to re-arrange. So the incoming wave will push electrons in a *very specific* direction. The motion of the electrons, which is very specific, creates a well defined electromagnetic wave.

So it's not like absorption that happens to an isolated atom or molecule. It is very different because it is absorption by the "sea" of electrons, which are free to move on a very large scale compared to atoms or molecules. Also, the time scale is different. The re-arrangement of the electrons in the metal is much, much faster than those in atoms or molecules. Since the process is longer in atoms/molecules, there is time for reorientation, and hence "random" emission of the absorbed light.

erm, nope, 'fraid that doesn't clarify for me at all, and you usually do Dealer!

In fact I can't even ask any sensible questions about it except to say it doesn't seem to explain why a photon entering at point A results in a photon emitted at point A. Nor why said photon is emitted on the trajectory it is.

One step too far for me I think...

Hey Dealer. I like that explanation. In fact I'll use it in class tomorrow.

Cheers!

Alex.

Well I hope none of your students are like me then!

Any chance you could translate it for me? I'm sur eit is very good, but I can't see it.

It probably is very good, Dealer usually is on these things, but it just doesn't click for me this time.

I'm not sure what you which part you want to know. I just like the bit about the sea of electrons and the positive ions not moving.

Alex.