Talk:Refractive index

From Wikipedia, the free encyclopedia

	Portal This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
???	This article has not yet received a rating on the assessment scale. [FAQ]
???	This article has not yet received an importance rating within physics.
Help with this template Please rate this article, and then leave comments to explain the ratings and/or to identify its strengths and weaknesses.

1 unheadered stuff
2 Quoted Indeces
3 Intro
4 What about Eta?
5 Superluminal speeds, and n<1
6 For the layperson
7 Definition Error
8 unhelpful definition
9 Stupid mistake
10 Negative Refractive Index - better explanation

[edit] unheadered stuff

From the article:

In a dielectric material such as glass, none of the light is absorbed and therefore k = 0.

This statement is wrong. While it's true that if light is not absorbed then k = 0, it's not true that in a dielectric material no light is absorbed. In this context does light mean visible light or electromagnetic radiation?

I have rewritten this part. It is true that perfect dielectrics (i.e. perfect insulators) do not absorb electromagnetic radiation at low frequencies, however at visible light frequencies there is always some dielectric loss due to polarization delays. R6144 08:03, 23 August 2005 (UTC)

www.ultra-faster-than-light.com

The above was added by an anonymous user. Hmmm... I wonder who. See http://groups.google.com.au/groups?th=ed639d47fcb6ca32 for some jaded responses to the website. -- Tim Starling

From article:

When light enters a diamond, the high refractive index causes it to suffer multiple total internal reflections, which is the reason for the brilliance of these gemstones.

Removed. The total interhjbvbnugvjnyjtjknal reflection is not special to diamonds, nor is is a natural property of diamonds. The stone must be cuts specially to show it, and the same can be done with other stones (most noticablly with cubic zirconia, and rock crystal). I can't think of a useful way yugftxerzw3azerdct6udtyswe4tZtcto put this that illuminates (har-har!) anything to do with refractive index.

!! Recommendation: phase velocity shbhy8g7yif76f54as4tezthfraqhjur76tfg5drse4d5rd4rd65dtydyould be named v instead of v, which does not differ from greek letter $ν$ Germendax 09:20, 3 Mar 2004 (UTC)

The problem is, it's standard in publishing and in Wikipedia to use italic letters for variables. The Tex-wiki markup does this when rendering as HTML, for instance. Anyway, the how distinguished v is from ν depends on your browser and which fonts you are using - they are quite clearly distinct on my setup (default IE6), for instance. -- DrBob

Why not use v_p for phase velocity and v_g for group velocity. This seems to be fairly common. Or, use c for phase velocity (reserving c_0 for the vacuum speed of light).

[edit] Quoted Indeces

I took the liberty of removing the incomplete table of refractive indices. It was the same as the one in list of indices of refraction, so it's still available. Incidentally, I don't think it's all that useful to quote indices for X-rays. Which wavelength do we pick? Tantalate 16:08, 16 May 2004 (UTC)

It is standard practice to quote the index at nD²⁰, that is the sodium 'D' doublet is used at 20 C. You will see such values tabulated as nD²⁰. --Askewmind | (Talk) 01:47, 15 Mar 2005 (UTC)

[edit] Intro

I'm no physicist, but this seems incorrect to me:

For a non-magnetic material, the square of the refractive index is the material's dielectric constant ε (sometimes expressed as the relative permittivity ε_r multiplied by the permittivity of free space, ε₀). For a general material it is given by:

$n=\sqrt{\varepsilon\mu}$

where μ is the permeability of free space.

First of all, isn't dielectric constant ε_r? Second, $\sqrt{\varepsilon\mu}$ , with μ the permeability of free space, can't apply to a "general material"; it takes no account of μ_r. Should this be $\sqrt{\varepsilon_r\mu_r}$ ? Josh Cherry 15:33, 17 Apr 2005 (UTC)

You are absolutely right. Askewmind | (Talk) 02:29, 13 May 2005 (UTC)

"General material" could be refering to material that is either nonmagnetic or nearly nonmagnetic. In these materials $\mu_r \approx 1$

[edit] What about Eta?

The refractive index is often written with the Greek letter Eta ( $η$ ). The article Eta (letter) refers to this, and links to the Refractive index page. It seems to be common practice to use the letter $n$ for the refractive index, but I think that might have stemmed from typographical inconvenience. I think it would be valuable to mention the different representations, and give evidence for the most common and most "correct" usage. The page on Snell's law uses $n$ too, and there are probably others. Does anyone have any thoughts? -- Andrew 00:05, 28 September 2005 (UTC)

It's possible that η was the original usage, and n came later. (I don't actually know who came up with the whole idea of refractive index - if I find out I'll add it to the article.) However, n is by far the most common symbol for it in optics. It's used everywhere, and in practically every textbook. -- Bob Mellish 16:24, 28 September 2005 (UTC)

[edit] Superluminal speeds, and n<1

From the short amount I read in the external link, I gathered that n can never be less than 1 for a *specific* frequency, IE n is only less than one for combined frequencies. Is this right? If so it should be User: fresheneesz

n can be less than one when the incident EM wave has a frequency higher than the resonant frequency of the atoms in the medium.

Is n<1 even possible? Because the velocity of light is given by v=c/n and no velocity can ever be greater than c. —Preceding unsigned comment added by 91.23.9.163 (talk) 06:49, 24 October 2007 (UTC)

[edit] For the layperson

Perhaps Wikipedia needs a new guideline: The intro to any scientific topic should answer the question, 'who cares and why do they care?' Refractive index is carefully defined, but help the layperson put it in context. —Preceding unsigned comment added by 66.92.53.49 (talk • contribs)

Better now? Han-Kwang 14:28, 26 March 2007 (UTC)

Excellent, nicely done. 66.92.53.49 02:22, 28 March 2007 (UTC)

Does light slow down?

The light waves that go through the glass don't actually slow down. The effect is only apparent and applies to the speed of light 'in the material' as opposed to the speed of light 'in vacuum' where light ALWAYS travels at the speed of light c.

The reason you treat the light as if it did slow down is an effect of the wave nature of light. We treat light as a wave, with a wavefront propagating with velocity c in vacuum. The wavefront represents a plane in which all the light waves are in the same phase. Now, when this wavefront hits a material, some of the wavelets will hit atoms and excite electrons to a higher energy state. Effectively, the electrons are 'swallowing' the light photon. Every material does this in a different way. The excited electron soon after releases the stored energy in the form of another photon. The key idea here is what happens to the phase of the wave as it gets absorbed and re-emitted. Depending on the resonant or natural frequency of the atom and the frequency of the incoming wave, the emitted photon will have changed phase when compared to it's unaffected brethren. It falls either back of forward a bit. The wavelet will do this every time it hits an atom, and there are quite a bit of atoms in even a small piece of material. This has the effect of retarding (or advancing) the wavefront as the wavelets go through the substance. The effect is most pronounced when the incoming waves are near, but not at, the resonant frequency of the material. At these frequencies, the change in phase lag (or change in effective wave speed depending on how you look at it) is great for a given change in wavelength. Most materials will have the effect of slowing the speed of the wavefront, but plasmas will actually speed it up. Notice the light wave is still only propagating at c, the phase velocity of the wave, however, may travel less, or even greater than at the speed of light.

[edit] Definition Error

The phenomenon This has practical technical applications, such as effective mirrors for x-rays based on total internal reflection. is actually called total external reflection. This allows for grazing incident mirrors to be used in x-ray optical systems, the light never enters the optic. --64.212.80.158 02:08, 28 June 2007 (UTC)

[edit] unhelpful definition

article (now corrected per Intro above) says in the Into

$n=\sqrt{\varepsilon_{r}\mu_{r}}$

This provides no insight. It's an index that used in calculations to ratio the speed of propagation of the wave in two medias. And it is itself the ratio of the speed of propogation between the media of interest and free space. It's better written as:

$n=\frac{\sqrt{\varepsilon\mu}}\sqrt{\varepsilon_{0}\mu_{0}}$

where

$v_{phase}=\sqrt{\varepsilon\mu}$

155.104.37.17 14:22, 11 August 2007 (UTC)

I would recommend against the use of $\varepsilon$ and

μ

. Many authors are rather sloppy as to whether they are dimensionless quantities (equivalent to $\varepsilon_r$ and

μ r

, as in Gaussian/CGS units) or in the correct (SI) way that you use. I've had enough trouble using equations from physical papers that were inconsistent in this respect, so let's not add to the confusion. Han-Kwang 14:45, 11 August 2007 (UTC)

It's certainly true that many authors drop the "r" subscript for relative permittivity. However, it is also true that the Wikipedia article should relate the refractive index to ε and μ. Omitting this relationship would be remiss; we just have to be clear about whether relative vs. absolute permittivity is meant. —Steven G. Johnson 20:14, 11 August 2007 (UTC)

My reflex about the the proposed equation is to rewrite it as:

$n=\frac{\sqrt{\varepsilon\mu}}\sqrt{\varepsilon_{0}\mu_{0}}=\frac{\sqrt{\varepsilon_l\varepsilon_0\mu_r\mu_0}}{\sqrt{\varepsilon_{0}\mu_{0}}}=\sqrt{\varepsilon_{r}\mu_{r}}$

so I don't really see the advantage. Whenever I encounter an absolute permittivity, I separate it into eps_0 and eps_r. But then, I'm not a theoretical physicist, so maybe you think there is a deeper truth to keep it in terms of the absolute permittivity. Han-Kwang 20:21, 11 August 2007 (UTC)

Sorry, I misread the discussion above; leaving everything in terms of the relative permittivity and permeability seems fine to me. The difference between relative and absolute is just a choice of units, and therefore fairly uninteresting in my opinion, as long as the choice of units is clear. (I am a theoretical physicist, but theorists just set c=1 most of the time anyway.) —Steven G. Johnson 20:28, 11 August 2007 (UTC)

[edit] Stupid mistake

"The macroscopic sum of all such contributions in the material is a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave's phase velocity."

Wave frequency and wavelength are dependent values, so that claim is impossible therefore stupid. —Preceding unsigned comment added by 128.139.226.37 (talk) 13:06, 12 September 2007 (UTC)

[edit] Negative Refractive Index - better explanation

It may just be my short attention span, but from the above section, I don't get what the EFFECTS of a negative refractive index would be. Anybody can make a try at explaining this a bit more? It certainly sounds like it couldn't mean that light suddenly gets quicker than light... Ingolfson (talk) 08:33, 21 April 2008 (UTC)

Categories: Unassessed physics articles