Light Speed in Glass & Second Postulate of Special Relativity

[zeithief]

wait. I'm a layman so i don't want to delve too deep into the equations. Some questions that i hope people will answer:
1) Since the speed of light apparently slows down in a medium, what does experiments that has been done by scientist on Earth (in air) tell us about the speed of light? What is the speed of light they recorded in their experiements? ( 3x10^8 m/s or the apparent slower speed predicted by snell's law?)
2) If they recorded the slower speed, how did they know that the speed of light has a 'real' value , that is the constant c?
Thank U!

 

[ZapperZ]

wait. I'm a layman so i don't want to delve too deep into the equations. Some questions that i hope people will answer:
1) Since the speed of light apparently slows down in a medium, what does experiments that has been done by scientist on Earth (in air) tell us about the speed of light? What is the speed of light they recorded in their experiements? ( 3x10^8 m/s or the apparent slower speed predicted by snell's law?)
2) If they recorded the slower speed, how did they know that the speed of light has a 'real' value , that is the constant c?
Thank U!

You need something considerably denser than air to significantly detect the deviation from the vacuum value. Even then, you need a very precise measurement. Case in point - you certainly do not alter your behavior during very humid weather than when the air is dryer, do you? Yet, in principle, light travels slower in more humid air. Such variations are practically undetectable.

Furthermore, in precision measurement, the presence of air is taken into account. Note that the index of refraction in a normal dispersive medium is frequency dependent - different frequency of light travels at different speeds. There have been several precise measurements to indicate that in vacuum, light speed is a constant for ALL freq., and even in air, such variation is hardly detectable.

Zz.

 

[Aether]

wait. I'm a layman so i don't want to delve too deep into the equations.

zeithief, your questions show that you are probably very curious about science, right? I think that you would be absolutely amazed at how satisfying a good equation can be to someone with this kind of curiosity (e.g., everyone reading this, including you).

 

[Garth]

zeithief, your questions show that you are probably very curious about science, right? I think that you would be absolutely amazed at how satisfying a good equation can be to someone with this kind of curiosity (e.g., everyone reading this, including you).

Well said!
And a very warm welcome to these Forums zeithief!
Garth

 

[yogi]

Originally Posted by ZapperZ
"Want more? Look at the isotope effect and its effect on the index of refraction! I can keep the same material, but increase the mass of the ions. This changes nothing on the field inside the material - you change nothing with regards to the charge of the atoms. Yet, this changes the index of refraction of the material."

Ah but it does - the electric field of the photon interacts with the electric field of the of the electrons in the outer shells - the electrons are bound by the nucleus - there is an inertial reaction - the inertial reaction depends upon the mass of the isotope. The heavier isotope will be displaced less so the inertial reaction acting back upon the photon will be greater - therefore it will be slowed to a greater extent

 

[ZapperZ]

Originally Posted by ZapperZ
"Want more? Look at the isotope effect and its effect on the index of refraction! I can keep the same material, but increase the mass of the ions. This changes nothing on the field inside the material - you change nothing with regards to the charge of the atoms. Yet, this changes the index of refraction of the material."

Ah but it does - the electric field of the photon interacts with the electric field of the of the electrons in the outer shells - the electrons are bound by the nucleus - there is an inertial reaction - the inertial reaction depends upon the mass of the isotope. The heavier isotope will be displaced less so the inertial reaction acting back upon the photon will be greater - therefore it will be slowed to a greater extent

But this is the PHONON effect, NOT a "field" effect. The field remains the SAME. I didn't change the nuclear charge nor the number of electrons. All I did was change the ionic MASS. Phonons are normal mode vibrations of the lattice. The optical mode depends on the displacement of the ions. I change nothing about the bonding strength, only the ions masses. The index of refraction changes because the response time of the ions are now different! The field strength that the photon encounters is still identical from before!

So your argument that it may be due to the matter field affecting the photon is wrong. It IS due to the ability of the lattice to oscillate fast enough to react to the photon - these are PHONONS.

I'd ask you to go look at phonon modes in solids, but I don't think you'd do that either.

Zz.

 

[yogi]

In crystalline substances, the displacement of atoms from their equiblibrium position leads to normal modes of vibration which, when superposed leads to a phonon lattice frequency. I am not saying there is a direct matter field effect that slows propagation - it is because the electon must displace its nucleus when perturbed by the passing photon - the greater the nuclear mass the greater the inertial reactive force acting back on the electron. This reactive force is coupled back to the photon through the electic field of the electron. Your phonon explanation is also dependent upon inertial reaction in that more mass coincides with a lower lattice frequency. So indirectly both arguments depend upon inertia.

Have you done any experiments with liquid mediums (light propagation in heavy water vs ordinary water)? In water and gas mediums, the phonon theory does not apply

 

[ZapperZ]

In crystalline substances, the displacement of atoms from their equiblibrium position leads to normal modes of vibration which, when superposed leads to a phonon lattice frequency. I am not saying there is a direct matter field effect that slows propagation - it is because the electon must displace its nucleus when perturbed by the passing photon - the greater the nuclear mass the greater the inertial reactive force acting back on the electron. This reactive force is coupled back to the photon through the electic field of the electron. Your phonon explanation is also dependent upon inertial reaction in that more mass coincides with a lower lattice frequency. So indirectly both arguments depend upon inertia.

Note what you said earlier that annoyed me:

Whether light travels in a medium at velocity c subject to delays due to absorption and re-emission as a new photon... or whether the photon itself is slowed by fields within a medium is not known.

1. Changing the mass of the ions WITHOUT changing the charge content does NOT change the field in the crystal.

Do you agree, disagree, or just do not understand this statement?

2. Graphite and Diamond are made of the same atom - carbon. There are no differences in "fields within the medium" for both material.

Do you argree, disagree, or just do not understand this statement?

In both (1) and (2), the index of refractions are different for the two difference cases. We KNOW why that is in terms of the material's STRUCTURE. The standard description invokes NO PHOTONS BEING SLOWED DOWN BY FIELDS.

Conclusion: we DO know what causes the reduction in the group velocity of light. Your claim that we don't know which one causes it is bogus. You may not know, but that's your problem.

Have you done any experiments with liquid mediums (light propagation in heavy water vs ordinary water)? In water and gas mediums, the phonon theory does not apply

And have you ever looked at molecular vibration modes in gasses and liquid? Phonon theory may not apply in those two phases, but "fields within a medium"?

You are still avoiding my question to you to quantitatively produce how much photons will be slowed down in such fields. The fields in matter is ridiculously small by comparison with the fields we have in particle accelerator. If they can be slowed down THAT much in matter, then I should be able to test this easily in my linac. Tell me by how much.

Zz.

 

[yogi]

Yes Zapper - i understand your questions - but I do not subscribe to the dictum of the standard model explanation. I agree that the static field is unmodified by the addition of isotope neutrons to the atom - but

Imagine a single photon passing near an atom having a single outer electron - the electric field of the photon intereacts with the electic field of the electron perturbing its orbit - the displacement of the electron is coupled to the atomic nucleus via the electric field between the positive charge of the nucleus and the electron causing - the nucleus to be displaced. The Reactionary force depends upon the mass of the nucleus - conservation of momentum requires the reactionary force to be felt by the electron via the coupling between the nucleus and the electron and the coupling between the photon and the electron - an atom locked in a crystal structure will be displaced less than in a fluid or gas - a lighter atom will be displaced easier than a heavy atom - the amount of force reflected back to the photon will depend both upon the rigidity of the configuration and the inertia of the atom.

 

[ZapperZ]

Yes Zapper - i understand your questions - but I do not subscribe to the dictum of the standard model explanation. I agree that the static field is unmodified by the addition of isotope neutrons to the atom - but

Imagine a single photon passing near an atom having a single outer electron - the electric field of the photon intereacts with the electic field of the electron perturbing its orbit - the displacement of the electron is coupled to the atomic nucleus via the electric field between the positive charge of the nucleus and the electron causing - the nucleus to be displaced. The Reactionary force depends upon the mass of the nucleus - conservation of momentum requires the reactionary force to be felt by the electron via the coupling between the nucleus and the electron and the coupling between the photon and the electron - an atom locked in a crystal structure will be displaced less than in a fluid or gas - a lighter atom will be displaced easier than a heavy atom - the amount of force reflected back to the photon will depend both upon the rigidity of the configuration and the inertia of the atom.

But this is all hand-waving! Figure out where the center of mass of the system is, and see BY HOW MUCH the nucleus is displaced due to this perturbing electron in your hydrogenic atom. I have $20 here that says the displacement is so small, we currently have no ability to detect it! So you are attributing an effect that is extremely noticeable (light slowing down in matter) to an effect that is so miniscule, we haven't detected it. Do you think you have made a rational hypothesis here? Couple this with no change in light speed when it passes through very high fields and I'd say you're up the creek without a paddle.

And why would this be relevant ALL THE TIME? In solids, the valence electrons are commonly either non-localized throughout the bulk of the material (metals), or in bonds being shared by 2 atoms (also non-localized between the two). The vibrational states induced is NOT due to the perturbing electrons, but rather the perturbing fields of the photon itself! That's why you have phonon modes and the "breathing" modes of molecules! The ions are displaced NOT due to the perturbation of the electron it is coupled to, but to the external field itself!

This bear repeating: atoms in solids and molecules CANNOT be considered naively as isolated atoms anymore! Such consideration will give hysterically wrong results that bears little resemblance to empricial evidence.

Zz.

 

[yogi]

Zapper: I would agree that the displacement of the nucleus is quite small - actually from the standpoint of a field effect, the smaller it is, the greater the reactionary force acting back on the photon (greater rigidity and higher mass corresponds to a higher index of refraction)

If as you state, the vibrational states are induced directly by the photon field - would not that have an effect upon the photon velocity - Is there not a momentum issue involved when the photon field sets off a vibrational effect. How can the photon influence a lattice or a gas or liquid molecule without itself not being influenced?

The electric fields near the electron or in the space between the electron and the central nucleus are very strong. Likewise with the magnetic field - for example The magnetic field due to the very small energy difference between the two sodium P states about 200,000 gauss.

We are still not communicating - you are claiming that the vibrational states are not due to the photons acting on the electrons - and that is as it may be - but I am saying, the vibrational states may only be a secondary effect ..that the main contribution may be field intereaction which does not require the absorption-release hypothesis.

Your view may be correct - or there may be theory that better explains light in a medium that depends upon factors yet to be discoverd - I simply have a skeptical view of the absorption theory as the correct M.O.

 

[ZapperZ]

Zapper: I would agree that the displacement of the nucleus is quite small - actually from the standpoint of a field effect, the smaller it is, the greater the reactionary force acting back on the photon (greater rigidity and higher mass corresponds to a higher index of refraction)

If as you state, the vibrational states are induced directly by the photon field - would not that have an effect upon the photon velocity - Is there not a momentum issue involved when the photon field sets off a vibrational effect. How can the photon influence a lattice or a gas or liquid molecule without itself not being influenced?

But the photon is no longer there! It has been absorbed by the lattice into its vibrational mode! Refer to Raman-type experiment! It is only when the lattice cannot sustain such vibrational mode (the optical or electronic band-gap is larger than the photon energy) is when the vibration reradiates this energy - thus, THE DELAY!

Again, you will get sick of hearing this, but this is a powerful technique in condensed matter physics to probe the phonon density of states of matter! I'm not making this up! The mechanism and process of photon interaction in matter is so well-known, it's not even funny anymore!

The electric fields near the electron or in the space between the electron and the central nucleus are very strong. Likewise with the magnetic field - for example The magnetic field due to the very small energy difference between the two sodium P states about 200,000 gauss.

And they have 20 Tesla field at the National High Magnetic Field Laboratory out in Florida. Want to bet if they see light changing its speed when they shoot it through the middle of their magnet borehole?

We are still not communicating - you are claiming that the vibrational states are not due to the photons acting on the electrons - and that is as it may be - but I am saying, the vibrational states may only be a secondary effect ..that the main contribution may be field intereaction which does not require the absorption-release hypothesis.

Prove it! Give me a QUANTITATIVE prediction of (i) the magnitude of the fields that can affect the photon speed and (ii) by how much the speed is reduced. I can then compare it to what we have, and what we have seen. Till then, you're just shooting in the dark.

Your view may be correct - or there may be theory that better explains light in a medium that depends upon factors yet to be discoverd - I simply have a skeptical view of the absorption theory as the correct M.O.

Let's see... we have used the "absorption" theory to study materials properties... and in fact, you are using many of them right now that benefited from such techniques... .. and the fact that they DO work, and the fact that you are using them reliably... Oh yeah, I can see why you're "skeptical" that they are correct. You'd rather we pay attention to your scenario that (i) has no quantitative predictions (ii) has never been shown to work.

Zz.

 

[yogi]

Zap - absorption theory in general and absorption/reemission theory as a rationale for the velocity of light in a transparent medium are different things. You might take a look at

http://www.sparknotes.com/physics/optics/geom/section2.rhtml

for another interpretation that does not involve absorption and reemission per se

 

[ZapperZ]

Zap - absorption theory in general and absorption/reemission theory as a rationale for the velocity of light in a transparent medium are different things. You might take a look at

http://www.sparknotes.com/physics/optics/geom/section2.rhtml

for another interpretation that does not involve absorption and reemission per se

Whoa! Read again!

When the "atom" re-emits (I always find it amazing that many explanation always make this mistake of thinking that "isolated" atom picture is valid in a solid) by causing the "vibration", where did you think the original energy of the photon go? It CANNOT still be around since (i) it still has the same energy and (ii) it has caused an energy transfer to the vibration! If the photon is still there, this is a VIOLATION of the conservation of energy right away! That vibration IS the absorbed photon's energy, thankyouverymuch! This is not "another interpretation". It's the SAME one.

And if you buy what is being descibed in this site, how come you ignored the part where they insist that inside the material, light still travels at c in between these processes?

Zz.

 

[yogi]

Zap - I didn't say this was my theory - its someone else’s - and I don't take issue with the fact that the photon could travel at c inside a medium until it encounters something - you postulate the promotion of an electron to a higher orbit - I believe the photon interacts with the atomic electrons and the nucleus - we do not have a good model of the electron or the photon - the wavelength of a green light photon is 5 to 10 times larger than the diameter of most atoms in transparent substances

Whatever the mechanism that causes slowing, it has to explain the velocity reduction in gases, liquids, crystalline solids and amorphous solids - so the problem reduces to finding the correct model for a single photon encountering a single gas molecule. We know that the photon is affected by G fields - inertial forces and gravity forces are equivalent - but inertial forces are approximately 10^11 times greater than G forces - so if there is any interaction that takes place when a photon encounters an atom, the reactionary forces could be significant.

 

[ZapperZ]

Zap - I didn't say this was my theory - its someone else’s - and I don't take issue with the fact that the photon could travel at c inside a medium until it encounters something - you postulate the promotion of an electron to a higher orbit - I believe the photon interacts with the atomic electrons and the nucleus - we do not have a good model of the electron or the photon - the wavelength of a green light photon is 5 to 10 times larger than the diameter of most atoms in transparent substances

I postulated WHAT? I did no such thing!

Here's something you need to learn. In a solid, the "isolated atom" model NO LONGER WORKS! So an electron being promoted to a higher orbit (whatever that is) is typically not true since the valence electron is no longer exclusive to just ONE atom! It is why you have bonds, and why there is a solid!

You "believe the photon interacts with the atomic electrons and the nucleus" is a fallacy. If you apply that belief and figure out QUANTITATIVELY (something you have been unable to do at all), you'll find hilarious results. Furthermore, what does the "wavelength" of photons have anything to do with the diameter of the atom?

It appears that all the discussion of lattice vibrations and phonons have gone in one way, and out the other way. Why we're back to atomic picture of absorption, I have no idea. Is this a red herring tactic on your part to not address your mistake in the previous posting?

Whatever the mechanism that causes slowing, it has to explain the velocity reduction in gases, liquids, crystalline solids and amorphous solids - so the problem reduces to finding the correct model for a single photon encountering a single gas molecule. We know that the photon is affected by G fields - inertial forces and gravity forces are equivalent - but inertial forces are approximately 10^11 times greater than G forces - so if there is any interaction that takes place when a photon encounters an atom, the reactionary forces could be significant.

But we ALREADY have a mechanism that explains the apparent slowness of the group velocity of light in matter - you are just ignorant of it. It matches QUALITATIVELY and QUANTITATIVELY the experimental predictions. We even USE this understanding in many situations already! If this understanding is wrong, we will be getting a set of puzzling observations.

You will notice that you still could not produce any quantitatively predictions to your claim that photons are affected by first the E-field, and then the B-field, especially after I pointed out that we routinely have lasers moving through very high fields in both types, way larger than what you would encounter in an atom. And now, you're wiggling out of this by introducing the "inertial forces" (whatever they are). This is typical trademark of someone who is making things up as they go along.

Since you like to state what you "believe" all the time, then let me say that I believe this thread is going nowhere, UNTIL you pick up a condensed matter book and figure out light transmission in matter.

Zz.