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Post by Progenitor A on Jan 9, 2011 10:44:06 GMT 1
Coming back to a point raised recently by Carnyx, why does light slow down in glass? Just what is going on?
I am not talking about refractive indices (at least not on th normal surface level) because that is simply a function of the slowing down. I am concerned to know what actually happens to the photons as they enter the glass.
The following explanation has been put forward but it raises so many puzzles that it surely cannot be right:
the photons excite the outer-valency electrons in the glass to a higher energy level. These electrons then fall back to their normal steady-state level and in so doing emit a photon; that is the process that slows down the light, the time taken between a light photon raising an electron to its higher level and the time taken for that same electron to fall to a lower level, emitting a photon.
Anyone have any idea what is going on?
No jeering at suggestions , guaranteed!
Note:
This is quite similar, I think, to Feynmans observation that when we stand at night in front of a window carrying a lamp we see a reflection of ourselves. No-one has a clue as to why some photons (of similar energy levels) are reflected and other go straight through the window.
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Post by speakertoanimals on Jan 11, 2011 14:25:46 GMT 1
I don't know where you got this daft explanation, but that is the problem, NOT the actual quantum theory that explains refractive index.
In quantum terms, we have excitations of the em quantum field in a vacuum (ie no real matter), compared to excitations when there is matter about. In quantum terms, the problem is totally different, hence really no surprise that solution should be different. The relevant quantum theory for the refractive index of gases goes back to at least 1958:
Phys. Rev. 110, 359–369 (1958)
Quantum Theory of the Refractive Index
C. A. Mead
This absorptiom and re-emission with time-delay supposed idea is nonsense, an attempted classical explanation, so no wonder it sounds daft....................
Within quantum theory, refractive index calculations using perturbation theory are standard, and they don't have a simple semi-classical explanation. In quantum terms, we have added interaction terms to the Hamiltonian (the interaction of the photon with electrons in the material), hence no surprise that the eigenstates are perturbed, which in physical terms means a refractive index.
Of course, you have to understand the basic of quantum theory before you have any chance of understanding the quantum explanation for refractive index...........................
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Post by carnyx on Jan 11, 2011 17:24:17 GMT 1
Oy, STA!
You deliberately stupid or what ?
A.
Then, you jeer;
B.
Then you say;
Then. it ain't very good as a general explanation, is it!
FACT... EM radiation is measurably slowed as it goes through glass.
Can you explain it? .... NO
And incidentally you are re-proving the idea that physics is becoming increasingly less powerful as it loses the ability to create extendable general explanations and so can give birth to powerful metaphysical ideas.
But I 'll say that modern physics HAS created the metaphysics that supports modern idiocracy.
And, if you carry on retreating behind this cloud of Quantum Gnosticism, you won't mind if the rest of the world ceases to fund this kind of idiot obscurantism on anything other than charitable grounds .
BTW = Are you a supporter of Postnormal Science, by chance?
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Post by speakertoanimals on Jan 11, 2011 17:35:35 GMT 1
Let's look at what could be going on when a photon enters a medium. we already have the suggestion in the first post that it is something to do with photons being absorbed then re-emitted.
Except that isn't really right -- we know that substances can absorb photons (and emit them), but that tends to be sharp spectral lines at definite frequencies, NOT the behaviour we get with actual optics, where refractive index changes slowly as a function of frequency.
So, what other processes can we imagine? From a classical point of view, a photon is a travelling and oscillating em field. An atom has electrons, which we might expect might be polarised by the passage of an em field. This sort of effect (unlike the very specific photon absorption between distinct atomic energy levels) would occur for a range of frequencies.
In effect, we have photon exciting electrons to oscillate, those electron oscillators will also radiate, and the combined effect of the whole she-bang is a photon that travels at a different speed to one where those electron-oscillators are not present.
This effect then depends on the density of polarisable atoms, and how easily polarised they are.
Its actually a bit more complicated than it might seem at first, in that the field trying to polarise each atom isn't just the field of the incident photon, but also effects from the polarisation of all the other atoms! We then get a basic result that involves the polarisability, and the density of atoms, which can be tested by looking at the refractive index of gases (where we can vary the density).
In QUANTUM terms, the relevant degrees of freedom aren't just the basic oscillators of the em quantum field, but as we can see from above, the effect we are looking for is a collective effect involving the basic em field AND the electrons in all the atoms. Remember that in quantum terms, we have to think of the photon as a wave, and the electrons as waves as well, and the net result of all that complicated, interacting waviness is a particle-like excitation that moves at a reduced speed compared to a vacuum. But it isn't a simple picture like a photon travelling along a definite path, binging off an atom here, getting delayed in its trip through the material -- the picture of something like a free photon existing between bings isn't quite right, just as when we have the double slit, the picture of a single photon passing through a definite slit isn't right either.
This polarisability picture works as a calculation assuming the effects are small. If you have light of a very particular frequency (ie close to a strong absorption line) passing through, then that is different again, and just considering a small overall polarisation of the electron clouds isn't good enough.
But here is a different take -- WHY is the net effect a slowing down? (let's ignore the difference between phase and group velocity, and weird negative refractive index and all that sort of stuff?).
Well, basic physics answers that -- we first have an em wave passing through NOTHING. If we put stuff in its way, then first guess would be it would slow it down. Second guess is that since the em field polarises atoms, the extra bit of field added by those polarised atoms acts to deter what first excited them (else we have a run-away situation that isn't physical!). Hence again, we would just expect that in general, net effect of putting stuff in the way of a em field would be to deter it from propagating, which in our terms would be a reduced speed.
Which makes it seem reasonable that the effect of stuff (if it lets light through at all, we haven't included effects such as photons actually getting absorbed and never re-emitted by the material!) in general is that it decreases the speed, and that there might be some slow dependance on frequency across the spectrum.
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Post by Progenitor A on Jan 11, 2011 17:52:27 GMT 1
What a shame that no-one wants to join this discussion. I was hoping that someone could provide insight into why/how light slows down in glass.
Seems , like me, no-one has a clue
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Post by carnyx on Jan 11, 2011 18:22:50 GMT 1
What I find fascinating is that light passing through glass does not apparently change frequency, but just slows down it's rate of propagation.
So, does light slow down instantly at the entry surface, and accelerate instantly at the exit surface?
Could the explanation be analogous to the passage of momentum through Newton's Cradle?
Is there a reduction in amplitude in all refractive media? As this would be a loss of wave energy, is there a correlation between refractive index and energy loss?
If so, what about material with a negative refractive index? Does such material speed up the rate of propagation.. i.e the light goes faster?
Is there a clearly defined physical characteristic, or property of material that is correlated with transparency? In other words, is it possible to predict transparency of a sample by knowing another characteristic? (e.g. are all gases transparent?)
And so on. An amazing subject, really.
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Post by Progenitor A on Jan 11, 2011 18:38:49 GMT 1
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Post by speakertoanimals on Jan 11, 2011 19:10:25 GMT 1
Wrong. What part of 'its standard quantum theory' did you miss, or does that count as 'not having a clue'....................
The change of speed (but not of FREQUENCY) is actually quite simple. Think of the classical case, of simple waves. We can imagine a constant field outside the material, and a different, constant field inside. no change in time.
Noww imagine a ripple on the field outside, one single ripple. It will cause a ripple on the field inside, but only after the ripple has reached the material! Hence on purely physical grounds, we can expect that what happens inside keeps pace with what happens outside.
If we have 3 ripples, one after the other spaced in time, we would expect 3 ripples inside, also spaced in time.
Hence the matching of frequencies follows totally naturally, difficult to see how it couldn't really, as long as the classical picture is a good approximation to the physics, as it is in the cases where a simple refractive index is sufficient to describe the net effect of the interaction of the em field with the material.
When it comes to acceleration -- the question doesn't make sense! So think again of my little ripple -- when it reaches the medium, we can imagine that a smooth ripple entering may change shape as it enters the material, that it may change speed, but can we really pin-down ONE point on the ripple and measure its acceleration? If we think about waves osciallating within a classical wavepacket, then again the question of acceleration becomes undetermined in the sense that we can't pin-down ONE point in an oscillating em field in that way.
Looked at in another way, we have to consider that what we mean by the boundary of the material becomes fuzzy as well. If we take a simple atomic solid, where is the boundary of the material? The idea that we have a definite boundary surface is just a useful approximation, hence that is where the seeming infinite acceleration comes from as well.
Yes, you have to consider the range of processes by which a material can absorb photons, which can be many and varied, from the scattering of photons at boundaries (why is glass transparent, yet you can't see through a bucket of glass chips?) on various scales, to absorption via excitations of the crystal lattice, to atomic and molecular (and even nuclear) single-atom absorption processes.
Reducing optics to a few simple, measurable things such as transparency and refractive index just tells us what happens on average -- if I send a billion photons of a particular frequency range at a material, how mnay get through, and how long do they take to make the journey compared to when the material isn't there. It doesn't really describe the whole range of things that might happen to a single photon, just the average effect. In simple cases, we ignore some effects (like absorption or scattering) in order to compute how long it takes to get through in a straight line for those that do get through.
Except since absorption and refractive index often refer to different physical aspects of the material, I would not expect so. Transparency, after all, can be reduced by just having MORE of the material (a thicker slab, or more dense gas), which doesn't change the electrical properties of the single atoms (polarizability) which is what is used in simple computations of refractive index.
YES. It's a bulk effect caused by the interaction of the em field of the incident radiation with the charges (in particular, electrons) which compose the material. If you knew what polarizability meant, you would have understood that.
It's actually quite simple. Take a simple atom, we have an electron (relatively light in terms of mass), bound to a heavy atom. Place the atom in an electric field, and the electron will tend to be pulled a little bit by that field, in an opposite direction to the nucleus (different charges move different ways in an electric field). That is the polarisation of the atom.
Disturbing the distribution of electric charge in the atom also produces an electric field, which tends to oppose the field you first applied.
To see what happens with em waves, you have to consider the time-varying em field you apply (the wave), the way all the atoms in the material respond to that field (the polarisation), and the net effect of the original field and the fields produced by the polarisation of all the atoms in the material. The net effect is a wave that moves with a reduced speed compared to when the material is not present. This can be explained with a simple classical model, although you need more complicated models if you want to explain exactly how a given atom responds to a particular applied field (ie if you want to explain why a certain substance has a particular polarisability).
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