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Speed of light in a medium
The current opening paragraph in this section seems rather confused to me and surely does not represent Feynman's words on the subject as the reference suggests. We currently have:
When light enters materials, its energy is absorbed. In the case of transparent materials, this energy is quickly re-radiated. However, this absorption and re-radiation introduces a delay. As light propagates through dielectric material it undergoes continuous absorption and re-radiation. Therefore the speed of light in a medium is said to be less than c, which should be read as the speed of energy propagation at the macroscopic level. At an atomic level, electromagnetic waves always travel at c in the empty space between atoms. Two factors influence this slowing: stronger absorption leading to shorter path length between each re-radiation cycle, and longer delays. The slowing is therefore the result of these two factors
This seems to mix up two models into one confused explanation. In classical electromagnetic theory, the original wave is extinguished by interaction with the bound electrons in the atoms of the substance and replaced with another wave moving at a lower velocity. This is a bulk phenomenon, representing the interaction of an EM wave with the huge number of electrons in the material.
- Where is the confusion? The EM-wave encounters a field of bound-charges. You then have a simple case of a field of driven oscillators. The new wave, travelling at c, is in turn extinguished like the original. As with any driven oscillation (with a mass) each generation undergoes a phase-delay - thus the phase-velocity of the propagation is reduced.
- You suggest that the 2nd generation wave propogates through the field of bound electrons without further interaction - which is not possible.
- I think your interpretation of the text above is somewhat charitable. I think that whoever wrote it was trying to explain the quantum behaviour of light in a medium in layman's terms, as TimothyRias says below. The text has been changed now anyway. Martin Hogbin (talk) 23:10, 15 June 2010 (UTC)
In QED photons are repeatedly absorbed and emitted by atoms resulting in a delay in the signal propagation through the material. As photons are quantum entities, I would stop short of saying that they travel at c between the atoms although this may be the mental model that many people have. Martin Hogbin (talk) 14:08, 13 May 2010 (UTC)
- Stopping short implies that photons might be travelling at some other velocity...
- I think the text is trying to explain the quantum behaviour of light in a medium in laymen's terms. Talking about virtual processes as if they were real always makes me cringe a little, but it is exactly the sort of thing Feynman does all the time. Someone with quick access to the Feynman lectures should check that this paragraph accurately reflects his explanation, though. TimothyRias (talk) 08:34, 17 May 2010 (UTC)
- No, IIRC, Feynman gives a purely classical explanation in that section (treating each electron as a harmonic oscillator, essentially; he only mentions at the end that for quantum reasons each one actually has several frequencies). He assumes an incoming wave on the left of a plane layer of transparent material, and describes the outgoing wave at the right a superposition of the incoming one and a wave generated by oscillating electrons, and the result has (at sufficiently low densities) almost the same amplitude but different phase than the original wave. If it's exp(ikx - i\omega t + i\phi_1) on the left, it's exp(ikx - i \omega t + i\phi_2) on the right, then he shows that \phi_2 - \phi_1 is proportional to the thickness, so that inside the material it can be written as (i(k+k')x - i\omega t + i\phi_1) and calls \omega/(k+k') + c/n. (When I get back home, I'll check whether it's actually as I remember it.) A. di M. (talk) 09:35, 17 May 2010 (UTC)
- Yes, I checked this out and Feynman is discussing the classical model in which the original wave is extinguished and a new one created.
- As I suggest above, I am not against giving a layman's QED answer but we must be careful not to describe photons as classical particles. Martin Hogbin (talk) 22:19, 18 May 2010 (UTC)
- I definitely agree on the last bit. Any ideas for a clear exposition? TimothyRias (talk) 07:46, 19 May 2010 (UTC)
- No, IIRC, Feynman gives a purely classical explanation in that section (treating each electron as a harmonic oscillator, essentially; he only mentions at the end that for quantum reasons each one actually has several frequencies). He assumes an incoming wave on the left of a plane layer of transparent material, and describes the outgoing wave at the right a superposition of the incoming one and a wave generated by oscillating electrons, and the result has (at sufficiently low densities) almost the same amplitude but different phase than the original wave. If it's exp(ikx - i\omega t + i\phi_1) on the left, it's exp(ikx - i \omega t + i\phi_2) on the right, then he shows that \phi_2 - \phi_1 is proportional to the thickness, so that inside the material it can be written as (i(k+k')x - i\omega t + i\phi_1) and calls \omega/(k+k') + c/n. (When I get back home, I'll check whether it's actually as I remember it.) A. di M. (talk) 09:35, 17 May 2010 (UTC)
I wish I had more insight. I don't know if I like any qualitative description of how the speed of light varies in a material before the article tells me which speed of light it is talking about. The mechanism for the phase speed is different then for the group speed and the front speed. My first instinct is that description fits the description of the front velocity and not the phase velocity since phase velocity can be and is for certain circumstances greater than c. The explanation for phase velocity almost certainly needs to involve waves since it is a wave phenomenon.
This article has progressed a lot since I last saw it, but this section is still its Achilles's heal. (The only other significant item is the amount of physics speak in the relativity section.) I will see if I can find something better somewhere. TStein (talk) 06:37, 22 May 2010 (UTC)
- As this article is not about light, it is somewhat out of the scope of the article to provide an explanation of why light travels at a different speed in a medium. I've been bold and rework the section to omit this, and focus on relaying the facts about light propagation in a medium. I've also added a good source discussing in detail the bussiness of group and phase velocities being superluminal or negative, and showing that the front velocity is equal to c. TimothyRias (talk) 15:48, 25 May 2010 (UTC)
- Looks good. I've copy-edited it a little. A. di M. (talk) 18:21, 27 May 2010 (UTC)
Am I interpreting this wrong?
'From the observation that the periods of Jupiter's innermost moon Io appeared to be shorter when the earth was approaching Jupiter than when receding from it, he concluded that light travels at a finite speed, and was able to estimate that would take light 22 minutes to cross the diameter of Earth's orbit.'
Light would take about 1/23 of a second to cross the diameter of the earth. What's this 22 minutes all about? —Preceding unsigned comment added by 91.108.4.98 (talk) 14:02, 29 May 2010 (UTC)
- The time needed to cross the diameter of the Earth's orbit. Not the time needed to cross the diameter of the Earth.TimothyRias (talk) 20:04, 29 May 2010 (UTC)
Misusing of refs
Jagged 85 (talk · contribs) is one of the main contributors to Misplaced Pages (over 67,000 edits; he's ranked 198 in the number of edits), and practically all of his edits have to do with Islamic science, technology and philosophy. This editor has persistently misused sources here over several years. This editor's contributions are always well provided with citations, but examination of these sources often reveals either a blatant misrepresentation of those sources or a selective interpretation, going beyond any reasonable interpretation of the authors' intent. Please see: Misplaced Pages:Requests for comment/Jagged 85. The damage is so extensive that it is undermining Misplaced Pages's credibility as a source. I searched the page history, and found 39 edits by Jagged 85 (for example, see this series of edits). Tobby72 (talk) 20:45, 14 June 2010 (UTC)
- From the edits above, the information that needs to be checked is the following text that is currently in the History section:
- Early Islamic philosophers initially agreed with the Aristotelian view that light had no speed of travel. In 1021, Islamic physicist Alhazen (Ibn al-Haytham) published the Book of Optics, in which he used experiments related to the camera obscura to support the now accepted intromission theory of vision, in which light moves from an object into the eye.
- This led Alhazen to propose that light must therefore have a finite speed, and that the speed of light is variable, decreasing in denser bodies.
- Also in the 11th century, Abū Rayhān al-Bīrūnī agreed that light has a finite speed, and observed that the speed of light is much faster than the speed of sound.
- I have seen the discussion of this editor's work, and there is wide consensus that enormous misrepresentations of sources have occurred, so this text needs to be confirmed. Johnuniq (talk) 23:55, 14 June 2010 (UTC)
- Well, the MacTutor and other sources quoted support the claims made almost verbatim. Since those sources are generally reliable the facts seem OK. The only issue remaining is the somewhat undue emphasis of the Islamic nature of these philosophers. TimothyRias (talk) 08:24, 15 June 2010 (UTC)
- Agreed. Are there any reliable sources suggesting that the religion of the philosophers was relevant to their scientific theories. If not the reference religion should be removed. Martin Hogbin (talk) 23:14, 15 June 2010 (UTC)
- Well, the MacTutor and other sources quoted support the claims made almost verbatim. Since those sources are generally reliable the facts seem OK. The only issue remaining is the somewhat undue emphasis of the Islamic nature of these philosophers. TimothyRias (talk) 08:24, 15 June 2010 (UTC)
- The point is the culture, not the religion. People of this culture are often referred to as Islamic (or Muslim) or Arabic (that is, by religion or by language) to distinguish them from other cultures that existed at about the same time in the same region. This should not be taken as an indication that the religion had much to do with the science. The early islamic philosophy article refers to Alhazen as an "Arab polymath"; some sources call him "Persian"; he was born in what's now Iraq and lived mostly in Egypt. Dicklyon (talk) 02:46, 16 June 2010 (UTC)
Are we ready for FA?
When I returned to this article last week, after about a month I hadn't significantly edited it, I found it even better than I remembered. I am quite positive that it is ready for FA status, but can anyone find any issue with it before I nominate it again? ― A._di_M. (formerly Army1987) 18:41, 10 July 2010 (UTC)
- I have an issue with the 'Fundamental role in physics section. In this section there important facts about the subject of the article have been relegated to footnotes. Why are they not in the main text? Martin Hogbin (talk) 23:40, 10 July 2010 (UTC)
- The Doppler effect and the Terrell rotation are quite irrelevant to the point being made, so I would keep them where they are. The Scharnhorst effect is so small that it might well be never observed during the lifetime of anyone around here, so it was agreed that it was undue weight to put it in the main text.
- Anyway, I'm going to move the reference to the relativity of simultaneity and the tachyonic antitelephone back into the main text (being sent to a footnote only comprising one link distracts more than seeing it in the text, IMO). As for the note on one-way vs two-way speed, I think it should be in the main text too, but I seem to remember there once was an opposition to that and was moved to the footnote as a compromise. ― A._di_M. (formerly Army1987) 13:56, 11 July 2010 (UTC)
Measurement
The introduction says about the speed of light: “Its value is exactly 299,792,458 metres per second”. Sydenham is cited in this connection, as is Jespersen “turning c into a conversion factor whose value is fixed and arbitrary” (p. 280). I'd suggest that some reconciliation of this viewpoint be attempted in the section Measurement which explains many methods for determining the speed of light and cites values like 299,710±22 km/s with error bars, as is appropriate only for an uncertain quantity, not an exact value.
I do not feel comfortable in pursuing a discussion of these matters, which caused me a great deal of difficulty with ArbCom in the past, and brought the most extreme invective and vituperation upon me that I have experienced in my 71 odd years of life.
However, it still seems to me that a naive reader is likely to wonder what is going on here. I hope that some enterprising soul can brave this wilderness to bring some clarity to the Measurement section. That involves at most the addition of a few sentences of reconciliation. Brews ohare (talk) 20:49, 15 July 2010 (UTC)
- As for the introduction, the reader is going to understand what is going on by the time they get to the end of the third paragraph. Maybe a paragraph could be added after the first paragraph of "Measurement" stating that today measuring c in metres serves no purpose (other than verifying that your measuring instruments are properly calibrated and properly working) and that measuring it in some other unit is equivalent to measuring the length of other unit in metres ... but I'm not sure of how to word it. What would you propose? —Preceding unsigned comment added by A. di M. (talk • contribs) 08:06, 16 July 2010 (UTC)
- A. di M.: I wouldn't hazard an attempt at wording given the history of conduct among editors of this article.
- The underlying difficulty with the Speed of light article is short shrift given to how a system of units based upon replacing ‘distance’ with ‘time-of-flight’ compares with a different system where distance and time are kept separate. That comparison could be done in a general manner for any speed standard, explaining the need for reassurance that the "standard" speed has been realized in any given measurement, and the role of definitions in making that speed "exact". That presentation could then be narrowed to describe why light-speed is a good choice. Brews ohare (talk) 11:36, 16 July 2010 (UTC)
- But there aren't two or more systems: modern physics is based on special relativity, from which we get that the speed of light is fixed, so distance can be defined in terms of time. Historically there were many different ways to understand it, all covered in the article, but the current system has been settled science for about a hundred years. The definition changed more recently, but standards bodies are much more conservative than scientists, and have to take account of how easily a definition is to use.--JohnBlackburnedeeds 13:53, 16 July 2010 (UTC)
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