Is the speed of light different in different medium?

Is the speed of light different in different medium?
In air, vacuum, water, is the speed of light constant or different?

 

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Yes and no. The propagation speed of light through different mediums will be less than c, but this is due to absorption and re-emission delays by the molecules in the medium. Between the molecules light still travels at c.

It is also important to note that whenever we talk about the speed of light being constant, we always mean 'c', the speed of light in a vacuum, and it is this value which is always used in Relativity.

 

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That's a good answer.

Cherenkov radiation is a phenomenon which happens as predicted because the speed of electrically charged particles (relative to a certain medium) are greater than the speed of light in that transparent medium, but still slower than the speed of light in vacuum. \(\frac{c}{n} \lt v \lt c\).

http://en.wikipedia.org/wiki/Cherenkov_radiation
http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/cherenkov.html
http://www.physlink.com/Education/AskExperts/ae219.cfm

A philosopher will see there are actually many types of photons.
1) Actual photons, as evidenced by the photoelectric effect and other experimental science.
2) QFT photons which obey the principal discovered symmetries of nature. These models of photons always travel at speed c and always are massless.
3) QFT virtual photons which conceptually explain QED momentum transfers as a coupling between the fermion quantum field and the photon quantum field. This model of photons necessarily subsumes the previous case. The success of QFT (and QED in particular) means that these first three senses are tangled up in most physics discussion, because when we talk of one we have no evidence we aren't also talking about the other two senses.
4) Photons as modeled in continuous media. These packets of energy and momentum effectively travel below the speed of light because we ignore the microscopic picture of sense-3 photons expiring upon contact with matter only to have identical photons spawn from that matter, relieving of the excess energy and momentum. Sense-4 photons follow from the properties of sense-3 photons combined with a study of matter, but sense-4 photons can also be studied in their own right.
That's rather a pedantic point, but one that is important to understand if one is to deal with the progressive nature of physics and how meanings of words like "particle" and "gravity" can change with one's choice of theoretical framework.

 

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No offense, but that's not really what's happening.

 

Hmmmm...Quite Interesting. I was going to concur 100% with Janus58, but I would like to hear a comment on at least the first video.

 

It is very much like the third model given in the first video. In order for light to follow every possible path through a medium its energy has to combine with the energy of the photons generated by that medium. These photons generated by the medium do have mass as the space between charged electrons and other leptons of the weak force could not be considered a true vaccum. This means a massless particle gains a portion of the mediums "relative density" If I may be so bold as to use an uncommon term to express something which is not quite a gain in mass. Hence a decrease in momentum.

 

These are shark-infested waters for a mere chemist, but here goes.

My understanding is that it is the polarisability of the medium which couples it to the photon and causes a sympathetic oscillation in it, which alters the phase velocity while maintaining the frequency. Thus, it is not really absorption and re-emission. That would be fluorescence and would cause the light beam to be scattered in all directions and in any case would only be possible at set frequencies, corresponding to electronic transitions within the atoms or molecules of the medium.

I think this is what the 2nd and 3rd explanation in the first video are both saying. I don't find the second video much good - I think the speaker has not really thought through exactly how he is going to communicate, before he starts, and the result is therefore not entirely clear.

An interesting side effect is that if one scans light of increasing frequency through a medium around one of its natural absorption frequencies, the refractive index becomes higher and higher, reversing as one passes through the absorption frequency (at which point the material becomes opaque of course) and then resumes with a refractive index <1 on the far side, rapidly returning to close to unity as frequency increases. It is compared to a forced oscillation, in which the light tries to drive the electrons at a frequency that is not quite right and they react with a phase lag or lead. At the transition itself of course one hits resonance and you get absorption! (Refractive index <1 does not violate relativity as it is only the phase velocity that exceeds c, not the signal velocity.)

But it's weird.

 

Agreed with most of your comments.

 

The way I look at it, I start with the classical electromagnetism explanation for refraction and then work it down to the quantum level from there. In the classical explanation, incoming electromagnetic waves stimulate the movement of charges and currents within the medium, producing secondary waves which interfere with the incoming waves in such a way that the net wave moves slower as a whole even though its constituent parts are all individually moving at the speed of light.

In quantum mechanics, those charge and current motions in the medium and the resulting polarization and magnetizations can be explained in terms of concepts such as the Stark and Zeeman effects. Instead of visualizing classical electromagnetic waves interfering with each other, you imagine photon probability waves being spontaneously generated and interfering with each other so as to delay the passage of the incoming photons. I don't think the simplistic picture of photons bouncing around between atoms adequately describes the situation, and it doesn't account for the ability to predict a medium's refractive index simply by knowing its polarization and magnetic susceptibility.

 

Mediums like glass, water, etc., will refract or bend the light. On the other hand, medium connected to GR (mass, gravity and space-time) will curve the light ? Could the curving of light, due to gravity, be thought of as 2-D refraction of light? Refraction in glass remains a 1-D path.

Along these lines, the speed of sound, for example, in water, is dependent on the temperature and pressure of the water. The speed sound in water (T,P) is only dependent on the medium. It is not dependent on the source of the sound or on relative motion in the medium. The medium itself sets a consist speed. This suggests the constant speed of light, in all references, is due to a consistent medium that propagates lights waves the same, regardless of the source, just like sound waves in the water medium.

When we have refraction, one could say we just added some salt to the water, such that the new composite medium has sightly different propagation properties, with this new composite medium also remaining constant in terms of the speed of sound/light.

The simplest way to model refraction, along this line of thinking, is the medium for light waves exists at the speed of light. This gives a consistent medium in our reference.

Since matter cannot travel at the speed of light, a medium composite forms. The two medium don't mix, with light still taking speed of light steps and matter steps. The analogy is like sound in an oil/water emulsion. With gravity and space-time the medium at C is confronted with another phase with the sound (path) curving. This increases the length of the path for any given radius of motion, from the source, to reflect the change of medium; amount of oil.

The old aether theory used the water medium analogy approach, but they were looking for a medium less than the speed of light, that we could measure. This was never found. But if the medium is at C, this solve the problem in the simplest way.

If we existed only at C, the finite universe would appear like a point-instant. If you could shrink yourself down to a point in the C reference, that only lasted an instant, we could still overlap the universe completely for its entire history. This unique situation gives the entire finite universe a consistent medium that lasts as long as the universe; one of the bookend of the universe.

 

How fast the electron spins around the nucleus?
Is it nearly the speed of light?

 

I'm not sure about orbital velocity, but in certain classical models of the electron, its has to be spinning around its own axis faster than the speed of light. In quantum mechanics, an orbiting electron with a well-defined energy state and angular momentum isn't moving with any velocity at all, it's more like a shifting cloud.

 

There is really no conflict between what the two professors say in their videos of post 4, that total ~20 minutes. Nor is there any error in Janus58's brief post; but you must know his "absorption" refers to ANY energy transfer from the "driving photons" to the matter (especially the electrons of it, which absorb almost all the energy transferred only to usually give it all back in "re-radiation") Often visible and lesser energy photon don't have enough energy to make the type of energy transfer absorption that transfers an electron from the ground state to even the first exited state of an isolated atom.

In dense matter there are "excited states" with much less energy required, even in the far IR that can sometimes do get excited but excitation by radio waves can't; however they can still do the "absorption" Janus58 was speaking of. I.e. heat matter - as your micro wave does.

Cut Janus58 some slack - he did not take 20 minutes of your time. It is my experience that Janus58 is one of the very few "error-free" posters here.

Now I want to amplify a few comment made in the videos about photon "going all possible paths." That is true of each and every photon, even if it (with very weak light source) is the only one that exists. I have used this to measure the length of some photons as ~30cm. I did not use very weak light source but like most, non lasers, the light coming from my source was incoherent yet I could display the expected interference pattern. (Because each photon ONLY interferes with its self.)

I used a Mach-Zener (Not spelled correctly?) two path interferometer. I.e. first thing the parallel beams from my extended source hit after passing thru lens one focal length away from the source was a "beam spliter" (half silvered flat piece of glass). There, completely in violation of the model that photons are particles, and violating your classical ideas based on your experiences, EACH photon "goes thru" and is also reflected by the beam splitter. Our two classical models of photons are "both wrong." Photons seem to be waves if you don't actually fully* absorb them; but it you do that, then all their energy is "dumped" at one very tiny place.

The next thing the photon (note this is singular) comes to in my is two mirrors set at 45 degrees to its path and in my case separated by about 4 feet. I.e. when reflecting by these mirrors (b & c in drawing below) the photon was 4 feet away from itself. Then both paths converge on a 2nd beam splitter, d, set at 45 degrees to the two possible paths and finally falls on the screen, s, with it very locally absorbed, or as screen was white paper, usually reflected as if radiated from one very tiny point of the screen. Man then used the "particle model" of the same photo he was earlier thinking of as a wave that could be spread out over more than 4 feet. Point is that the "All possible paths" that Feynman and the videos speak of is not just some man-made model as an aid to understanding and calculations; it is the REALITY OF NATURE - THE WAY PHOTONS ARE. Mother nature does not care that this is hard for you, with your only classical experience, to believe. She does as she likes with no regard for your confusion as to how to describe her.

* When you scatter a photon it does give tiny "recoil energy" to the scatter so that both energy and momentum are conserved but the photo is just slightly "red shifted" not made to behave like what man calls a particle.

Posting now below a drawing (in two parts) showing proof that one photon can "spread out" over more than four feet. "Feeling" a parts of a piece of glass as spoken of in the videos is "no stress" on it.

Extended light source and lens making parallel beams (0nly one shown below) but each part of the source makes a beam at very slightly different angles:

*
*
*...............................................................()===== This beam enters beam splitter "a" shown below (this part of drawing separated for ease of construction.)
*
*

Ok, that is best I could do. (If I made lens () taller then parts of the light source, represented by some * , would be too far above or below lens.)
Below is one of the slightly divergent beams (only one shown), leaving the lens and going to first 45 degree beam splitter "a" and going straight thru with part (of same photon) going up to hit 45 degree mirror, b, too, which makes it again traveling parallel to the entering beam.
Sorry that these beam splitters and mirrors are not shown actually at 45 degrees - but that is best a "typed drawing" can do.

.........................................................d
..........................b/======/======.....This is the path of "self- rejoined" photon to the screen thru another lense one focal length from it.
............................||..........................||
............................||..........................||
............................||..........................||
............................||..........................||
()===== / ======/c
Lens.....................a

Optically an "extended source" with lens one focal lengh from it followed by a second lense one focal length from the screen, just images the source (up side down) on the screen. Inserting these beam splitters and mirrors does not change that. It only make it possible for slightly differing path "split photons" to arrive at the screen where they would have but now they "want" to get back in phase with them selves, and do so as best a they can. Leaving dark lines where if they can not become "particles" there as they would not exist there since their waves are 180 degrees out of phase with themselves there. Note almost all the time photons can be thought of as waves, but it detected / absorbed they "die as particles" in one spot - not spread out over miles as they can be when waves.

Note that the length of paths: abd and acd, are the same. I.e. when the separate SINGLE photo get back together with itself, after being at times in its flight it 4 feet from itself, it arrives at the screen, unified, at the same time, but some of the many slightly divergent beams arriving there are "out of phase" with themselves and cancel (make dark interference lines) on the screen. The diverse in angle beams following paths abd & acd are exactly the same length ONLY for paths with pairs of equal angle degree corners. (parallelograms or rectangles.)

Now here is what you do to measure the length of a photon: You rotate beam splitter a very slightly counter clockwise, so that the path ab passes to the left of mirror at b, but pull mirror b back to still be hit by that now tilted beam. You of course must also rotate mirror b slightly clockwise, so the beam leaving it follows the old path to beam splitter d again. Now the corner turned at b is not 90 degrees. Perhaps this adds 5 cm of extra length to path abd.

What one sees on the screen is that there is a little light where there was none. I. e. the interference pattern on the screen is a little "washed out." This slight twisting of a & b is increased and then the pattern is more washed out. I kept repeating this until with ~30 cm extra path length for abd, the screen was with uniform illumination.

Crudely speaking this implies that none of the part of the photon going via path abd had yet arrived at the screen before the full length of the part of the same photon going by path acd had already disappeared into the screen. I.e. my spectral line source was making photons that were about 30cm long.

 

Actually the average outer electron in a smaller atom moves roughly 10% of the speed of light. However, in heavier atoms, like Gold, the electrons move much faster to create pronounced relativistic effects. An area of science called, Relativistic quantum chemistry deals with the relativistic electrons in heavy atoms. Relativity is used as an adjustment to classic wave functions to account for various properties that wave functions don't predict properly.

The yellow color of gold is connected to a relativistic time shift in the reflected light due to the relativistic speed of the outer electrons in gold. Gold's outer electrons are sort of in another reference (time/space delay), making gold very stable with respect to reacting to smaller atoms like oxygen. Platinum is also very stable and inert at higher temperatures, but has the property of being able to absorb hydrogen. Hydrogen can bridge the relativistic reference gap.

Theoretically, hydrogen and platinum could be used to time and distance shift electrons to release energy, with the electrons then speeding back up due to the heavy platinum nucleus. Some of the early cold fusion may have been using relativistic phase shifting; energy shifting in time and space.

 

As I recall, your first paragraph is basically correct. After that you go astray, I think. For example the bit about why gold is yellow. I would need to check the electronic shell structure of periodic table but bet there are at least 18 other elements that have electrons in that same "outter shell" that "move" in a classical sense at the same speed as gold's outter electrons do, yet don't have gold's yellow color. For example the two elements adjoining gold in the periodic table are platinum and mercury. Neither has gold's color but without checking I think their outer shells differ from gold's by one more or one less electron only.

Gold is, I think, the most malleable solid.* Sheets of gold only a few dozen atoms thick can be made. If you hold one up the white light source, you can see thru it to the source, but it looks green. I.e. Green wave lengths falling on a gold brick penetrate deeply into it and few of those photons manage to get sattered back 180 degrees to come to your eye. Thus the light that a gold birick reflects back to your eye is white light minus the approximately green wave lengths - we say that light has a "gold color."

* I recently had a slice of dark chocolate cake in a fancy restaurant. Sprinkled on top of it were a few flakes of gold. (An attractive contrast with the chocolate.) Gold sheets, when thin as they can be beaten, are probably the lowest cost sheets that exist, per square meter. The flakes I ate, were each less than 2mm square in area. I expect they were originally part of a larger sheet beaten so thin that it became tiny separate pieces. I doubt I ate even one cent worth of gold.

PS while Janus58 has error free set of posts, you tend to have always at last one serious error in each post! - That is what happens when you just make things up.

 

BTW, exchemist is fully correct in his post 7 too. Many physicist could not do as well - many don't know what he tells in the last paragraph. If there are any sharks in the water where exchemist is fishing, they had better head for the bottom and not bite anything Exchemist is offering.

In my post 15, I commented that the wave model of photons is best most of the time, but when they die, they die as particles in one very small (atomic size or less) spot. CaptBork and others were concerned about speed of an electron in orbit about a nucleus. That "speed" is sort of a particle concept, not even a valid concept for a standing wave

I think it was Bohr who modeled the energy levels of a hydrogen atom with waves going around and their "heads" being required to over lap IN PHASE with their "tails." Sort of a "standing wave" idea or at worst a slowly drifting standing wave.

He had a value for the "wave length of an electron." If one full cycle of it is "wrapped" around a proton, then that circumference is a specified distance from the point proton. - A certain way up the negative potential hill which he could calculate. I think the answer is 13.6 ev. from being free (ionized) for the one wavelength around case.

Like wise if two full cycles of the electron wave length are "wrapped" around the proton, then the circumference is twice as big and twice as far from the proton. He could calculate how deep in the negative potential well that would be and got 13.6 /4 ev.

For three wave lengths wrapped around that depth in the well from being free is 13.6/9 ev and in general the negative energy levels go as 13.6 n-2. ev. with n being the number of full electron wave lengths that wrap around to come back in phase with themselves. A concept that does give the hydrogen energy levels correctly!

So, once again, as I said, the electron is a wave until you kill it - just like photons are until they dies (are fully absorbed, not just slightly red shifted by a scattering , like "Compton scattering" does.) So that energy and momentum conservation are honored by "mother nature."

BTW, in post 10 wellwisher runs true to form (confused, wrong, and counter factual) with this idea:
"medium also remaining constant in terms of the speed of sound/light."
If that were true, then a prism shaped piece of glass would not spread white light beam entering one side into various different colored beams emerging from the other.

 

I have not fully read any wellwisher post for ages. I give them a general quick look over, and in near all of them, I find the tiny tit-bits of true knowledge and error free sentences, is grossly overwhelmed in the extreme by the rampant philosophical claptrap and personal weird takes on any situation that follows.

 

I think it's best to think of the photon as something like a seismic wave. If a seismic wave propagates from A to B, it isn't just the houses sitting on the AB line that shake. Hence many-paths. As for all their energy being "dumped" in one very tiny place, it seems to resemble the optical Gourier transform, check out Stephen Lehar's web page:

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Your "fat finger" hit the G instead of the F but I knew immediately from first glance at the figure what you were posting about and corrected my quote of you. It is fun to have second lens one focal length way from the plane of the transform and then a focal length from that lens another screen which recreates the source (except for components that diverged more than the first lens intercepted.

The "fun" (and some education) comes with the burnt black head of a match stuck on a needle that can selectively blocks /removes one component of the FT from the reconstruction of the original image. I found that the more interesting original images to do this with had 2D regularity - a piece of screen wire is very interest to use as the original. Not only can you remove components from the transform plane, but you can warp its regularity too.

Also with a set of regular parallel grids you can get quite quantative evaluation of the nature of the lens defects - spatially resolve those defects into components.

 

I think you both have the better explanation. The vids interested me but your explanations are better.

If we think of refraction as a bulk effect acting on the wave velocity then we can almost dispense with trying to trace the path at the level of the atomic interstitial space. Actually in the first vid the prof is saying glass, which is almost always amorphous, but he's drawing crystalline material. Minor point, but it serves to show the advantage of keeping this at the bulk level.

I would have just said that whenever the impedance changes, the phase shifts and hence the velocity changes. I'm pretty sure I can calculate the characteristic impedance if I know the polarization and susceptibility, so I think I'm on the right track. And there is a dual for the velocity change for acoustic waves in air vs water which uses the stiffness (or compliance) and density (I think) which are duals of something like permittivity (or polarization) and permeability (or susceptibility)-- from which we can calculate a characteristic impedance for both of them.

So using your better explanations as the gold standard, I'm betting that if we reduced this to a single parameter it's probably impedance.

Also, velocity (esp c) can be written as the inverse of the geometric mean of permittivity and permeability. And they establish the impedance.