Talk:Refractive index

From Wikipedia, the free encyclopedia

WikiProject Physics This article is within the scope of WikiProject Physics, which collaborates on articles related to physics.
Portal:Physics
 Physics Portal
??? This article has not yet received a rating on the assessment scale. [FAQ]
??? This article has not yet received an importance rating within physics.

Please rate this article, and then leave comments here to explain the ratings and/or to identify the strengths and weaknesses of the article.

Contents

[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 internal 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 to put this that illuminates (har-har!) anything to do with refractive index.

!! Recommendation: phase velocity should 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 εr 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 εr? Second, \sqrt{\varepsilon\mu}, with μ the permeability of free space, can't apply to a "general material"; it takes no account of μr. 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.
Retrieved from "http://en.wikipedia.org../../../r/e/f/Talk%7ERefractive_index_e939.html"