Talk:List of refractive indices

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This list is meaningless and misleading and, I suspect, wrong in many cases. For one thing, you have to specify a wavelength in order to give the index, since all realistic materials are dispersive (i.e., index varies with wavelength). Second, you must cite your sources, especially since measurements of indices aren't always the same to three decimal places between experiments. Third, some of the materials are just plain wrong. For example, copper (a metal, which will have a complex index with very large magnitudes in both the real and imaginary parts, at least for visible wavelengths) is listed with an index of 1.675, the same as copper oxide! Ditto for aluminum (perhaps alumina was intended?). Steven G. Johnson 22:52, 8 Dec 2003 (UTC)

Note that these criticisms applied to the list of indices in the original version of this article. Steven G. Johnson 23:06, 17 Mar 2004 (UTC)

PS. A good reference for this sort of thing would be the Handbook of Optical Constants of Solids by Palik.

A very large table of simple values is avaliable at http://www.robinwood.com/Catalog/Technical/Gen3DTuts/Gen3DPages/RefractionIndexList.html . Could some expert comment or post a link to it on the article? Thank you.

Unfortunately, this list is meaningless because it doesn't give the wavelength(s). Since it is for 3d modelers, I'm guessing it is intended to be at optical wavelengths, but in this case its metallic indices seem questionable to me. Since the page gives no reference, it is impossible to know where he got them and how accurate they are (or, indeed, if they are just fudges to get good-looking results in ray-tracing tools). —Steven G. Johnson 22:52, Nov 5, 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) 01:40, 24 Mar 2005 (UTC)

That may be common practice, but that doesn't mean you can assume any random indices someone posts on the Internet, without citing sources, are at that wavelength. —Steven G. Johnson 18:07, Mar 24, 2005 (UTC)

Very true, Steven, I was not trying to say we don't need references, wikipedia has no credibility since we all can write whatever we want. I was mainly considering the fact that we need some sort of consensus whe we point the refractive index of substances. References are always required. I recommend Sigma-Aldrich for most of the common chemicals. Askewmind | (Talk) 18:23, 24 Mar 2005 (UTC)

The entries for HgS, GaAs, and Si do not make a lot of sense because they are either poorly transparent in the visible range, or not at all. So, yes, they have high indeces of refraction, but no lens for the visible range would be made from them. The values can be used to estimate surface reflectance off the air-material interface, but the imiginary component becomes important if it's not pretty small. Could a new column with the imiginary index value be added to the table? Metals are very complicated, the two index components can change a lot over the visible range, and are best shown with a plot, not just one data point. 131.252.124.185 22:05, 29 September 2006 (UTC) David

This list would be more useful if every value listed had some citation that gave a link to relevant and trustworthy external material. These numbers have little meaning taken out of context. Each value needs additional information about both the material and the wavelength. Selain03 20:51, 2 February 2007 (UTC)

1 Complex Number?
2 neuron?
3 wavelength or frequency ?
4 Crystal

[edit] Complex Number?

How does a pair of two different physical properties--refractive index and absorptivity--constitute a complex number? Was this concept taken from scientific literature, or from the mind of a creative contributor? It reminds me of Hawking's "complex time". I would rewrite this myself, but I see the person who added it is a professor of Applied Math and has a background in Physics, so I'll leave it up to him.

It is completely standard; look in any textbook. Because Maxwell's equations are linear (for linear materials), it is correct and convenient to write oscillatory fields with a time-dependence exp(-iωt) instead of with cos or sin (with the understanding that the "physical" solution is the real part of this). Then, in a uniform medium, the spatial dependence becomes exp(iωx * n/c), where n is the index. Thus, the real part of n gives an oscillatory term, and the imaginary part gives an exponential decay. Equivalently, if you look at how the conductivity σ of a material enters Maxwell's equations, it can be equated with an imaginary dielectric constant ε=iσ/ω for time-harmonic exp(-iωt) fields. —Steven G. Johnson 17:22, Apr 7, 2005 (UTC)

[edit] neuron?

Index of refraction for neuron. A nerve cell, is this correct?

[edit] wavelength or frequency ?

Someone said that through refraction, it is the speed (so the wavelength) which modifies, not the frequency. If that's so, I think it would be more accurate to give the frequency of the wave as related value, not the wavelength.

When people give a wavelength associated with an index of refraction, they normally mean the frequency associated with that wavelength in vacuum. (You're right that the wavelength changes inside a material, whereas the frequency is preserved. However, the vacuum wavelength is much more conventional to give at infrared and higher frequencies. A note about this in the article would be nice, or perhaps just a link to vacuum wavelength.) —Steven G. Johnson 21:50, Jun 10, 2005 (UTC)