Light Speed in Glass & Second Postulate of Special Relativity

In summary, the second postulate of Special Relativity states that the speed of light in a vacuum remains the same for all inertial reference frames. However, in a medium such as glass, the speed of light may vary depending on the observer's velocity and the medium's speed. Relativity still applies, but the constant c must be the speed of light in a vacuum. This means that someone inside a medium cannot use the speed of light in that medium as c. Instead, the constant c must be the speed of light in a vacuum.
  • #1
Dr.Brain
538
2
Second Postulate Of Special Relativity:
"Speed of light in vacuum remains same for all inertial reference frames"

The speed of light in glass is 2 x 10^8 m/s . So how is now the above postulate applicable?...Is it true that if one was sitting inside the glass slab , he would see the light's speed to be 'c'? and not 2 x 10^8 m/s ? ...

Is the slowing in speed of light apparent?...just because we are sitting outside it but the person inside the glass slab would see the correct light speed>??..

And suppose person A is inside the glass slab and person B is outside the slab , that is in vacuum ..and there is a light beam which goes across in the vacuum ..now what will be the light speed for person A? ...for B the light's speed will be faster outside the glass slab and slower inside the glass slab , but for person A in the glass slab , what will be the case??

and how is all of the above in accordance with the second postulate?

shall i say " Light speed in glass remains same for all inertial frame observers inside glass slab" ..?
 
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  • #2
Please don't talk about refractive indices ..I know how to caclulate light speed in different mediums...I just want to connect it all with relativity ...how is second postulate applicable?
 
  • #3
You can treat the speed of light in glass just like any other speed except c. It will vary with the observers speed, and it will vary with the medium´s speed.
All relativistic effects follow from the constancy of the spped of light in vacuum and are not applicable to any other speeds which don´t remain constant.
 
  • #4
So you mean relativistic effects are not applicable inside a medium but only in vacuum?...I don't think so.
 
  • #5
The person in the glass slab will also see light in the glass slab travel slower than light in a vacuum. If both the person in the glass and the person outside the glass are at rest relative to each other, they will agree on the speed of both light beams. The person in the glass slab will say the light beam in the vacuum moves at c, while the light beam in the glass slab moves at some velocity < c, and the person in the vacuum will agree with this.
 
  • #6
Dr.Brain said:
So you mean relativistic effects are not applicable inside a medium but only in vacuum?...I don't think so.

Relativity will still be applicable, but someone inside a medium can't use the speed of light in his medium as c; the constant c must be the speed of the propagation of light in vacuo.
 
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  • #7
If the person in the glass is small enough, he'll see light traveling at the same speed as it does in a vacuum - but every now and then interacting with a molecule. The net effect of that interaction on the macroscopic scale is an apparent - but not real - slowing of the speed of light.
 
  • #8
russ_watters said:
If the person in the glass is small enough, he'll see light traveling at the same speed as it does in a vacuum - but every now and then interacting with a molecule. The net effect of that interaction on the macroscopic scale is an apparent - but not real - slowing of the speed of light.

That means if the person is inside the glass slab , he will see the light speed as 'c'?

εllipse said:
Relativity will still be applicable, but someone inside a medium can't use the speed of light in his medium as c; the constant c must be the speed of the propagation of light in vacuo.

I think you mean in the last line 'glass' and not 'vacuo' ..?
 
  • #9
Dr.Brain said:
I think you mean in the last line 'glass' and not 'vacuo' ..?

No, I didn't mean glass. c is the speed of light in a vacuum, regardless of what medium you are in. Light propagates in air slower than in a vacuum, but even though we are in the air, c is still the speed of light in a vacuum, and the speed of light in air will be slightly less than c, as measured from both us and someone in space.
 
  • #10
The second postulate is applicable because it specifcally talks about the speed of light in a vacuum. Clearly to be internally consistent the velcoity of light through a medium must transform in the same way as any other velocity, so no the speed of light through a medium is not generally constant. That's all that really need be said as though SR references light in it's postulate it is not a theory of light in itself.
 
  • #11
I believe jcsd has correctly given the answer. I just wanted to add that the refractive index of a material (and hence the speed of light in that material) is frequency dependent.

So if red light and green light go from vacuum into glass... They have the same speed c in vacuum, but when they enter the glass they'll have different speeds... so speed of light is not constant in a medium.
 
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  • #12
Dr.Brain said:
That means if the person is inside the glass slab , he will see the light speed as 'c'?
No, he'd have to be around the same size as a molecule and be able to watch light go from molecule to molecule at C, then pause as it is absorbed and re-emitted by each molecule.
 
  • #13
  • #14
russ_watters said:
No, he'd have to be around the same size as a molecule and be able to watch light go from molecule to molecule at C, then pause as it is absorbed and re-emitted by each molecule.

So that means that if we are outside the glass slab , we are large enough that we don't account for the photon-level speeds but all we see is the net effect of photon being obstructed in their path by the molecules of the medium , as a result we apparently see that light has slowed down .
 
  • #15
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. One can construct a theory along the lines of an interaction between the electric field of the photon and the electric field of the electrons in the material. This is tangentially related to the question of whether photons fall like particles in a gravitational field or whether they only change frequency when moving from one gravitational potential to another.
 
  • #16
yogi said:
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.

Sorry, but this IS known! Lena Hau and her students just didn't go into their labs blindly and manage to slow down light via trial and error without any clue on what they should manipulate. We also know enough to construct metamaterial to not only make photonic band gap materials, but to make them have negative index of refraction!

The study of optical conductivity in matter is a very well-known subject in condensed matter physics. It is so well-known, in fact, that we use this to study the properties of materials. Do not associate what you do not know with the state of knowledge in physics.

Zz.
 
  • #17
Dr.Brain said:
So that means that if we are outside the glass slab , we are large enough that we don't account for the photon-level speeds but all we see is the net effect of photon being obstructed in their path by the molecules of the medium , as a result we apparently see that light has slowed down .
Yep...and since high schools never go beyond that when teaching it, it leaves many with a false understanding of what is really going on.
 
  • #18
russ_watters said:
Yep...and since high schools never go beyond that when teaching it, it leaves many with a false understanding of what is really going on.

Same problem with me.I was only taught about snell's law and how light slows down , never knew about the inner mechanism.Thanx for the new knowledge.
 
  • #19
SR says all frames with constant relative velocities are equivalent.
That is not true in a glass plate. SR is irrelevant inside matter.
Any EM calculations must be done in the rest frame of the glass.
Then, any questions about what happens if the glass moves, can be answered by LT.
This is how the Fizeau water tube experiment is explained.
 
  • #20
If photons are slowed down because they are absorbed and re-emitted won't they come out in all sorts of random directions?
 
  • #21
If photons are slowed down because they are absorbed and re-emitted won't they come out in all sorts of random directions?

Ever saw a glass of water?.. Under normal conditions, the molecules on the surface of the glass are in jiggling motion. Some of them constantly leave the surface while some other molecules of other gases in air join the surface. Its a pretty crowdy process where everyone is trying their best to be stable.

And when all of the above is goin on , what do our poor eyes see? ...all we see is the rubber-like disciplined-surface not moving at all.

The net effect is always plain and simple but when the details are deciphered , you will notice a crowd of movement.
 
  • #22
Zapper "Sorry, but this IS known! Lena Hau and her students just didn't go into their labs blindly and manage to slow down light via trial and error without any clue on what they should manipulate."

I am familiar with her experiment - there are a lot of back and forth reflections generated by the cross lasers in the Einstein concentrate as well as information retention which she contends is an effective slowing of the individual photon - it is an interesting experiment but does not obviate the dependence of photon velocity upon local electrical and inertial fields - we already know that G fields affect photons - atmospheric light scattering depends upon the interaction of the electric field of the photon with the electons in the atmospheric atoms - Try and explain why an em wave is slowed in a transmission line composed of a super conducting center wire and a superconducing outer shell separated by a vacuum.
 
  • #23
Dr Brain, you are quoting me but I'm not sure if what you have written is meant to be an answer to my question?
 
  • #24
gonegahgah said:
Dr Brain, you are quoting me but I'm not sure if what you have written is meant to be an answer to my question?


Just like you thought that as each and every photon is absorbed and then emitted , as a result as per you, light should be a bit scattered but inversely the light looks pretty clean and narrow, similarily as on water surface everything looks pretty simple but the molecules are making a lot of crowd.I was quoting an example.
 
  • #25
yogi said:
Zapper "Sorry, but this IS known! Lena Hau and her students just didn't go into their labs blindly and manage to slow down light via trial and error without any clue on what they should manipulate."

I am familiar with her experiment - there are a lot of back and forth reflections generated by the cross lasers in the Einstein concentrate as well as information retention which she contends is an effective slowing of the individual photon - it is an interesting experiment but does not obviate the dependence of photon velocity upon local electrical and inertial fields

But you're missing the point. It may not "obviate" anything, but it certainly doesn't mean we know nothing about it. We apply what we know already to the situation, and it explains it PERFECTLY well, both qualitatively and quantitatively. Your claim we know nothing about this is bogus. Furthermore, the cross laser has NOTHING to do with affecting the transmission laser - it is used to control the MEDIUM.

What she did is CONSISTENT with how we now light is transmitted in optical conductivity studies of matter. Look at FTIR and Raman scattering studies, for instance. Or look at the Optical sum rule that is often used in optical studies. They are both consistent with each other on how light interacts in matter! It is WELL-KNOWN, not unknown!

- we already know that G fields affect photons - atmospheric light scattering depends upon the interaction of the electric field of the photon with the electons in the atmospheric atoms - Try and explain why an em wave is slowed in a transmission line composed of a super conducting center wire and a superconducing outer shell separated by a vacuum.

1. Because superconductors do NOT have zero resistivity for AC current; 2. Because the cooper pairs do not have zero mass; 3. Because the group velocity and phase velocity in ANY waveguide depends on the geometry of the boundary conditions and the group velocity of the wall currents! You are forgetting that many people work in superconducting RF cavities, especially for accelerator accelerating structures (Look at work done at Cornell and the TESLA facility in Germany). In NONE of these have there anything done similar to what you are implying. In fact, I've just written a research proposal to build our own superconducting RF cavity!

We know enough to work with it and to use it to study other things. Again, do not equate your ignorance of optical conductivity in condensed matter physics with the state of the current knowledge.

Zz.
 
  • #26
Zapper: Where did I say "we don't know nothing about it" What is evident is that the Experimenters went to great lengths to control the situation to prevent the results that would occur if the photon were captured and released at random - In State 1 the cooled atoms are reduced to the ground state with the valance electron in its lowest orbit with spin anti-aligned with B. State 2: the electron and nuclear spins are aligned and in state 3 the valance electron is kicked up to the next orbit - if a photon pulse is injected into a condensate without the electromagnetic transparency provided by the cross laser, the atoms would absorb all the photons and elevate from state 1 to 3 and then later relax by emitting photons randomly in all directions. The Frozen Light experiment doesn't support the proposition you have cited - case in point - it teaches away from the notion that the slowing of light in a medium is consequent to absorption and re-emission - the theory that atoms in a solid or gas absorb and re-emit photons in precisely the right direction and at precisely the right time requires many ad hock postulates to make it work - it violates the uncertain principle as well as the statistical nature of the emission process.

AS an aside - you can sent a unidirectional em pulse (step function) down a transmission line- you do not have to use ac
 
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  • #27
gonegahgah said:
If photons are slowed down because they are absorbed and re-emitted won't they come out in all sorts of random directions?
No - conservation of momentum requires that they be emitted in the same direction as they were absorbed, for the most part.
 
  • #28
yogi said:
...the theory that atoms in a solid or gas absorb and re-emit photons in precisely the right direction and at precisely the right time requires many ad hock postulates to make it work - it violates the uncertain principle as well as the statistical nature of the emission process.
That's a cowpie.
 
  • #29
There's so many misinterpretation here, I don't even know where to start.

yogi said:
Zapper: Where did I say "we don't know nothing about it" What is evident is that the Experimenters went to great lengths to control the situation to prevent the results that would occur if the photon were captured and released at random - In State 1 the cooled atoms are reduced to the ground state with the valance electron in its lowest orbit with spin anti-aligned with B. State 2: the electron and nuclear spins are aligned and in state 3 the valance electron is kicked up to the next orbit - if a photon pulse is injected into a condensate without the electromagnetic transparency provided by the cross laser, the atoms would absorb all the photons and elevate from state 1 to 3 and then later relax by emitting photons randomly in all directions. The Frozen Light experiment doesn't support the proposition you have cited - case in point - it teaches away from the notion that the slowing of light in a medium is consequent to absorption and re-emission - the theory that atoms in a solid or gas absorb and re-emit photons in precisely the right direction and at precisely the right time requires many ad hock postulates to make it work - it violates the uncertain principle as well as the statistical nature of the emission process.

Note WHY I brought up the Lena Hau's experiment in the first place. When you said

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.

... you're equating your ignorance about light transmission in matter with the state of knowledge of physics. If this process is REALLY not known, then Raman scattering is hocus pocus. I'd like to see you publish something to debunk the whole field of optical conductivity in condensed matter by arguing what you just said.

And I would strongly suggest you figure out the typical strength of the field in a medium to be strong enough to affect a photon so much as to 'slow' it down. You cannot just make a handwaving argument like that without any quantitative predictions - this isn't physics! Figure out the typical strength of field in a crystal (hint: use the crystal field splitting value).

Now if you think the field is strong enough, then compare that with the typical field strength that *I* work with, which is of the order of 90 MV/m (yes, that's "mega") and tell me why *I* detect NO slowness of the photons that I shoot through such a field. This field is SEVERAL orders of magnitude of any possible field that can be set up in a crystal/matter.

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.

I could go on and on and tell you about how the index of refraction depends on the crystal structure (diamond versus graphite), etc. But somehow, I have a feeling you're not going to get these either.

AS an aside - you can sent a unidirectional em pulse (step function) down a transmission line- you do not have to use ac

Since when does an "em pulse" not contain an oscillating E-field component? Even in a traveling wave structure, you STILL have both E-field and B-field components that oscillates. You will continue to have oscillating charges in the walls!

Zz.
 
  • #30
ZapperZ said:
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.
The specific refraction [tex]R=[(\eta^2-1)/(\eta^2+2)]\cdot (1/\rho)=\frac{4}{3}\pi \cdot (N_A/M)\cdot \alpha[/tex] of a particle (where M is the molecular weight) is substantially invariant under changes in density. Multiplying through by [tex]\rho,\ (\eta^2-1)/(\eta^2+2)=\frac{4}{3}\pi\cdot n\cdot N_A\cdot \alpha[/tex] where n is the number density. Clearly, the molecular weight of the particle divides out and has no significance.
 
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  • #31
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!
 
  • #32
zeithief said:
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.
 
  • #33
zeithief said:
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).
 
  • #34
Aether said:
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
 
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  • #35
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
 
<h2>1. How does light speed in glass differ from its speed in a vacuum?</h2><p>The speed of light in a vacuum is a universal constant, known as c, which is approximately 299,792,458 meters per second. In a transparent medium such as glass, the speed of light is slightly slower due to interactions with the atoms and molecules in the material. This is known as the refractive index, and it varies depending on the type of glass.</p><h2>2. Why does the second postulate of special relativity state that the speed of light is constant?</h2><p>The second postulate of special relativity, also known as the principle of constancy, states that the speed of light in a vacuum is the same for all observers, regardless of their relative motion. This is a fundamental principle in Einstein's theory of relativity and has been confirmed by numerous experiments.</p><h2>3. Can light ever travel faster than its speed in a vacuum?</h2><p>No, according to the second postulate of special relativity, the speed of light in a vacuum is the ultimate speed limit in the universe. This means that nothing, including light, can travel faster than c. This has been supported by countless experiments and is a crucial part of our understanding of the laws of physics.</p><h2>4. How does the speed of light in glass affect the perception of time?</h2><p>According to the theory of relativity, time is relative and can be affected by the speed of an object. When light travels through a medium such as glass, its speed is slower, which means that time appears to slow down for an observer in that medium. This is known as time dilation and has been demonstrated in experiments with particles traveling close to the speed of light.</p><h2>5. Does the speed of light in glass have any practical applications?</h2><p>Yes, the speed of light in glass has many practical applications, particularly in the field of optics. The refractive index of different materials, including glass, is used to design and manufacture lenses, prisms, and other optical instruments. The speed of light in glass also plays a crucial role in fiber optics, which is used for high-speed data transmission in telecommunications and internet technology.</p>

1. How does light speed in glass differ from its speed in a vacuum?

The speed of light in a vacuum is a universal constant, known as c, which is approximately 299,792,458 meters per second. In a transparent medium such as glass, the speed of light is slightly slower due to interactions with the atoms and molecules in the material. This is known as the refractive index, and it varies depending on the type of glass.

2. Why does the second postulate of special relativity state that the speed of light is constant?

The second postulate of special relativity, also known as the principle of constancy, states that the speed of light in a vacuum is the same for all observers, regardless of their relative motion. This is a fundamental principle in Einstein's theory of relativity and has been confirmed by numerous experiments.

3. Can light ever travel faster than its speed in a vacuum?

No, according to the second postulate of special relativity, the speed of light in a vacuum is the ultimate speed limit in the universe. This means that nothing, including light, can travel faster than c. This has been supported by countless experiments and is a crucial part of our understanding of the laws of physics.

4. How does the speed of light in glass affect the perception of time?

According to the theory of relativity, time is relative and can be affected by the speed of an object. When light travels through a medium such as glass, its speed is slower, which means that time appears to slow down for an observer in that medium. This is known as time dilation and has been demonstrated in experiments with particles traveling close to the speed of light.

5. Does the speed of light in glass have any practical applications?

Yes, the speed of light in glass has many practical applications, particularly in the field of optics. The refractive index of different materials, including glass, is used to design and manufacture lenses, prisms, and other optical instruments. The speed of light in glass also plays a crucial role in fiber optics, which is used for high-speed data transmission in telecommunications and internet technology.

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