Lower speed of light inside a transparent medium

[George K]

Hi everybody. I had a discussion on another (engineers') forum about the "lower than c" speed of light inside a transparent material (e.g. glass). The explanation that I gave is that the reduction of the light's speed inside such a material is only "apparent" and not "real". The incident photons are absorbed and re-transmitted by the material's atoms (i.e. an atom's electron is stimulated on a higher energy level -via the absorption of the photon's energy- and, right after that, it returns to its fundamental energy level, by transmitting another photon). In this way the light propagates inside this medium. However, between the atoms the light still travels at speed c (because only vacuum exists between the atoms). So, the "lower than c" speed of light inside this medium is caused by the overall delay due to this absorption and re-transmission process by the atoms. So, only the average speed of light is lower than c. Is this explanation correct?

 

[drvrm]

However, between the atoms the light still travels at speed c (because only vacuum exists between the atoms). So, the "lower than c" speed of light inside this medium is caused by the overall delay due to this absorption and re-transmission process by the atoms.

Your simple explanation is correct.
The denser the material, the more atoms there are in the way of traveling light..
The more number of atoms it encounters , the slower the effective average speed.

 

[ZapperZ]

Hi everybody. I had a discussion on another (engineers') forum about the "lower than c" speed of light inside a transparent material (e.g. glass). The explanation that I gave is that the reduction of the light's speed inside such a material is only "apparent" and not "real". The incident photons are absorbed and re-transmitted by the material's atoms (i.e. an atom's electron is stimulated on a higher energy level -via the absorption of the photon's energy- and, right after that, it returns to its fundamental energy level, by transmitting another photon). In this way the light propagates inside this medium. However, between the atoms the light still travels at speed c (because only vacuum exists between the atoms). So, the "lower than c" speed of light inside this medium is caused by the overall delay due to this absorption and re-transmission process by the atoms. So, only the average speed of light is lower than c. Is this explanation correct?

You might want to try reading this first:

https://www.physicsforums.com/insights/do-photons-move-slower-in-a-solid-medium/

Also note that this isn't really due to "atomic absorption", at least for most ordinary cases. You might also want to look up the concept of "group velocity". It isn't strictly an average velocity.

As a reminder to new members to the forum, you might want to look at our FAQ as a start, and the Insight section.

https://www.physicsforums.com/threads/physics-faq-list.855542/

Zz.

 

[ogg]

Just to echo ZZ and disagree with drvrm. Simple Quantum theory predicts that photons not absorbed will pass by an atom without interacting and only photons of very specific wavelength/frequency/energy will be absorbed. Your idea of an atom eating everything, but almost instantaneously spitting out photons of any energy not "tasting" right {not to put words in your mouth :) } is very wrong. Most engineers can take a couple of chemistry courses then forget the quantum mechanics, and consider atoms as an electric field "balls" which, like macroscopic objects, have definite shape and size. Even though you probably learned that this wasn't the "correct" model when you drilled down into the sub-atomic physics, it's good enough for almost all day-to-day purposes (chemistry, biology, friction, wear, strength, fatigue,...) but this wouldn't work for some electrical and nano-material engineers who deal with things at atomic dimensions... Anyway, consider aluminum foil and window glass - there's more mass in a pane of glass than in a same area foil of aluminum. Al is LOWER in at.wt. than Si, O, K or Ca so you've got a problem. and I won't even mention the problem with how mirrors reflect light...
ALSO - be sure to read the comments in the explanation ZZ provides, I also believe "electric field polarization" is better than phonons at explaining decreased velocity, but these guys are over my head...

 

[sophiecentaur]

The incident photons are absorbed and re-transmitted by the material's atoms (i.e. an atom's electron is stimulated on a higher energy level -via the absorption of the photon's energy- and, right after that, it returns to its fundamental energy level, by transmitting another photon)

This is popular misconception and it's due to the fact that the Hydrogen atom is where we start learning about QM. That process only works for isolated particles. When EM waves hit condensed matter (solids/ liquids) the molecules act as a whole and coherently. If the mechanism was as you suggest, each atom would absorb and then emit photons at random times and in random directions and the resulting wave from all the photons would no longer be formed in any particular direction.
You have to go from Simple Atomic Physics to Solid State Physics, to understand what happens in a solid. The wave interacts with a whole region in the solid and, that way, it passes through in a coherent form without being split up and just scattered in a simpler model.

 

[George K]

Thank you all for your kind replies. Although I presented this explanation I had my doubts for its validity (e.g. I couldn't explain why the incident light is traveling inside the transparent medium at a specific angle instead of just being scattered in random directions).
To ZapperZ: I read your explanation. I have some questions:
1) Is this the official explanation of the refraction? (I, also, read the comment -as Oog suggested- about the "exciton-polaritons" explanation. Is this an "alternative" explanation" or what?)
2) Is this explanation, also, valid to the reflection?
3) The "debate" that I had in this other forum was "if the photons' speed is actually reduced inside a transparent material". I claimed that the actual speed of light is not reduced, but rather "delayed". Of course -as you pointed out- my explanation (for this delay) was wrong [I was one of the victims of this misconception :-)]. But, as far as I understood from your article, the photons are -kind of- temporarily absorbed & re-transmitted not by the atoms themselves but rather by the whole lattice of the solid material (i.e. the lattice cannot sustain the vibrations caused by the specific photons so it re-transmits these photons). So, between these "processes" the photons still travels at speed c inside this material. Am I right?

Best regards

George

 

[sophiecentaur]

the actual speed of light is not reduced, but rather "delayed".

There is no need to put too much into the value of c, which only applies in a vacuum. The same effect of slowing down a sound wave can be seen in gases in which it is inversely proportional to the root of the molecular mass or the waves propagate along a string of masses and springs.
It's always problematical when you want to know 'what actually happens and why' in a phenomenon. The mechanism of wave propagation can be described but that isn't what you seem to be after.

 

[George K]

Yes, I'm looking for the real reason why the photons are traveling in a lower speed inside transparent medium. Between the atoms of the lattice there is nothing else than vacuum. So in these "vacuum areas" the photons must still travel at speed c. I suppose that, again, it's only the average speed of photons that is lower than c (during their travel inside this medium) due to the delay caused by their interactions with the lattice (i.e. with the lattice as a whole and not with the individual atoms as ZapperZ pointed out). Am I right?

 

[George K]

You might want to try reading this first:

https://www.physicsforums.com/insights/do-photons-move-slower-in-a-solid-medium/

Also note that this isn't really due to "atomic absorption", at least for most ordinary cases. You might also want to look up the concept of "group velocity". It isn't strictly an average velocity.

As a reminder to new members to the forum, you might want to look at our FAQ as a start, and the Insight section.

https://www.physicsforums.com/threads/physics-faq-list.855542/

Zz.

Hi ZapperZ. I saw this link and your explanation. Is this an official explanation of the phenomenon?
(Please, take a look at my post#6 in this thread.)

 

[jbriggs444]

Yes, I'm looking for the real reason why the photons are traveling in a lower speed inside transparent medium. Between the atoms of the lattice there is nothing else than vacuum. So in these "vacuum areas" the photons must still travel at speed c.

Photons are not little bullets. They do not have a well defined attribute of "position". It is presumptuous to speak of them traveling through vacuum areas. The real world is more bizarre than you suppose.

 

[George K]

Photons are not little bullets. They do not have a well defined attribute of "position". It is presumptuous to speak of them traveling through vacuum areas. The real world is more bizarre than you suppose.

Hi jbriggs444. Of course the photons' position is not precisely known before any observation (we can only estimate their probable position via quantum physic's tools). But the light always consists of photons despite of our "ignorance" of their exact position. By considering the light only as a wave before any observation or measurement doesn't solve the problem. It's just a convenient approach. I mean, OK, we can explain the behavior of light by the well known behavior of a wave. But the photons are always over there and we cannot just ignore them.

You said: "The real world is more bizarre than you suppose."
Hmm... your statement just depicts our ignorance of how the nature really works...

 

[jbriggs444]

Hi jbriggs444. Of course the photons' position is not precisely known before any observation (we can only estimate their probable position via quantum physic's tools). But the light always consists of photons despite of our "ignorance" of their exact position.

It is not that photons have an exact position that we cannot determine. They do not have an exact position at all. It is not that we do not understand how nature works in this respect. We understand it. But it turns out not to conform to our everyday intuitions. A model of photons as little bullets with exact but unknowable positions does not conform to reality.

 

[phinds]

But the photons are always over there and we cannot just ignore them.

No, as jbriggs just pointed out, you MUST ignore then until you do a measurement since they have no position until measured. Light does not propagate as photons, as you seem to believe, but as waves. It interacts as photons if you are doing a measurement that tests for photon behavior.

 

[George K]

No, as jbriggs just pointed out, you MUST ignore then until you do a measurement since they have no position until measured. Light does not propagate as photons, as you seem to believe, but as waves. It interacts as photons if you are doing a measurement that tests for photon behavior.

Hi phinds. Yes, I know that this is the official Quantum physics (Copenhagen) interpretation. However, it always bothered me that we should consider the light as wave before any measurement and as particles (photons) right after the measurement. For example, let's consider the two-slit experiment. By sending out light (i.e. plenty of photons) we get a clear picture of the interference fringes on the screen depicting the wave nature of light. However we can send out just one photon. In this case we get just one point on the screen depicting that this photon has arrived and has been detected. The image (i.e. a dot) doesn't depict any wave behavior of this single photon. In order to get the interference fringes we have to emit a large amount of such photons (even one-by-one). So, I think that it's also "arbitrary" to assume that a single photon (right after the emission) turns itself into a wave and then (right after its detection) again turns itself into a particle. As an "alternative interpretation" we could assume that the single photon remains a particle during its whole travel and that the "wave properties" accompanies and characterizes its behavior.

 

[sophiecentaur]

Hi phinds. Yes, I know that this is the official Quantum physics (Copenhagen) interpretation. However, it always bothered me that we should consider the light as wave before any measurement and as particles (photons) right after the measurement. For example, let's consider the two-slit experiment. By sending out light (i.e. plenty of photons) we get a clear picture of the interference fringes on the screen depicting the wave nature of light. However we can send out just one photon. In this case we get just one point on the screen depicting that this photon has arrived and has been detected. The image (i.e. a dot) doesn't depict any wave behavior of this single photon. In order to get the interference fringes we have to emit a large amount of such photons (even one-by-one). So, I think that it's also "arbitrary" to assume that a single photon (right after the emission) turns itself into a wave and then (right after its detection) again turns itself into a particle. As an "alternative interpretation" we could assume that the single photon remains a particle during its whole travel and that the "wave properties" accompanies and characterizes its behavior.

This is turning into one of those dreadful and totally unsatisfactory "what is it really?" threads. You are on a hiding to nowhere when you assume that you can describe the real fundamentals about anything in Science. Physics is all about Models that Work. The rest is speculation or Philosophy.
If you feel happiest with little bullets that are hidden from view in a sort of wave, until someone reveals where the bullets happen to be then that is fine but you are working at one level too deep to have a sensible conversation about that on PF. As long as the Maths that you are using is the same as it is for the Wave theorists then you will all predict the same results for any experiment. That's as much as anyone can ever expect.

 

[George K]

It's not a matter of maths, it's just a way of interpretation. Quantum Physics doesn't claim that there are no photons (or that they are "transformed" to something else) before any observation, i.e. the light still travels in the form of photons. Of course, we can only estimate the probable position of each of these photons (and not their exact position) *. If we still believe that the light still travels inside a medium in the form of photons, then it's unavoidable to conclude that, in the vast vacuum areas between the atoms, the photons should still travel at a speed c. So the apparent reduced speed of photons should be the result of the intermediate (and continuous) interactions with the atoms of the lattice (although these interactions are more complicated, probably in a way that ZapperZ has presented).

[* I like to believe than -in a deeper level of reality- the exact state of the photons are well defined even before any observation. Our "lack of knowledge" about their state may arise from a "lack of information", i.e. the nature reveals only a part of the complete information of their state to us (as a kind of "censorship").]

 

[sophiecentaur]

[* I like to believe than -in a deeper level of reality- the exact state of the photons are well defined even before any observation. Our "lack of knowledge" about their state may arise from a "lack of information", i.e. the nature reveals only a part of the complete information of their state to us (as a kind of "censorship").]

They are fine as personal beliefs but they really have no place in discussions of Physics. Heisenberg is pretty well established and you are going against it, for a start. You seem to be expressing on a religious belief about the World. Who is doing this 'censoring', for instance? Is it not just that we are part of the 'system' so we cannot possibly understand it fully? If you try to draw a map of your locality then the map has to be included (And you) in it and on that map, will be information about the whole locality - including the map etc. etc.. Physics (all Science) is concerned with drawing as much of that map as it can, given the limitations of human intelligence, time and money. That's all.

 

[Nugatory]

Part of the problem here is that you are starting with some basic misconceptions about the treatment of light in quantum physics. Electromagnetic interactions, which include everything involving photons and their interaction with matter, are described by the theory of quantum electrodynamics, and you have to use the methods of QED instead of ordinary non-relativistic QM in these problem. For example, all of the following statements are things you might conclude for yourself from reading non-mathematical treatments that kinda sort of apply to the behavior of massive particles - but the mental picture you've formed is seriously misleading when applied to massless photons.

But the light always consists of photons despite of our "ignorance" of their exact position.

Yes, I know that this is the official Quantum physics (Copenhagen) interpretation ... that we should consider the light as wave before any measurement and as particles (photons) right after the measurement.

Quantum Physics doesn't claim that there are no photons (or that they are "transformed" to something else) before any observation, i.e. the light still travels in the form of photons

Without working through a serious textbook on QED, it's going to be hard to improve on ZapperZ's response in post #3, so this thread is closed.