# Light In A Medium

#### Dr.Brain

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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#### Dr.Brain

Please dont 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?

#### Ich

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.

#### Dr.Brain

So you mean relativistic effects are not applicable inside a medium but only in vacuum?....I dont think so.

#### εllipse

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.

#### εllipse

Dr.Brain said:
So you mean relativistic effects are not applicable inside a medium but only in vacuum?....I dont 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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#### russ_watters

Mentor
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.

#### Dr.Brain

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' ..?

#### εllipse

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.

#### jcsd

Gold Member
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.

#### learningphysics

Homework Helper
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 vaccuum 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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#### russ_watters

Mentor
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.

Staff Emeritus
Gold Member

#### Dr.Brain

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 donot 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 .

#### yogi

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.

#### ZapperZ

Staff Emeritus
2018 Award
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.

#### russ_watters

Mentor
Dr.Brain said:
So that means that if we are outside the glass slab , we are large enough that we donot 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.

#### Dr.Brain

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.

#### Meir Achuz

Homework Helper
Gold Member
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.

#### gonegahgah

If photons are slowed down because they are absorbed and re-emitted won't they come out in all sorts of random directions?

#### Dr.Brain

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.

#### yogi

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.

#### gonegahgah

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?

#### Dr.Brain

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.

#### ZapperZ

Staff Emeritus
2018 Award
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.

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