Talk:Relative permittivity

Mirostrips (section title added)

Can anyone talk about the effective dielectric constant which is used for computation in microstrip?

You can find alot of usefull information here and here. This information should be included in the microstrip article, and this discussion should be moved there. Karol 06:54, 14 October 2005 (UTC)[reply]

Table of values

I moved the table od dielectric constant values from permittivity, and formatted it a bit. Karol 09:38, 29 November 2005 (UTC)[reply]

I think that the table of dielectric constant values is a bit confusing for a typical reader. It shouldn't be necessary to learn how to read the table. For an example "128 aq–60". Is it "128 aq - 60" or "128; aq-60". More semicolons or even text would fix it. 1exec1 (User talk:1exec1) 15:32, 16 August 2008 (UTC)[reply]

The table's caption reads "Relative static permittivities ... under 1 kHz", but if it's static, it can't be frequency-dependent. So we should remove either the "static" or the frequency information, "under 1 kHz". Looking into the source [1], there is no mention about frequency, suggesting that these values are indeed static. What would you think?

Yeah, I was confused as well... The table says "under 1 kHz" so I assumed it was typical/average/approximate values for low frequencies, but it lists values for "visible light" (for water) which would be well into the THz range, and one for 1MHz, so I'm not sure what to make of the rest of the data, nor how reliable any of it is. I think a less comprehensive table would be more helpful even if it had more restrictive parameters, or better yet, separate tables for different ranges. I'll try to see if there is a better one in the appendices in one of my old physics textbooks when I have time. kumowoon1025 (talk) 18:50, 28 January 2018 (UTC)[reply]

The table lists "High Polymers" and "Sodium Nitrate" as very high premittivity materials. I have searched for information on both of these materials and can find no evidence they are. Please provide a reference for these values.

Thanks Elmerfud (talk) 18:31, 14 December 2007 (UTC)[reply]

oh yes, and nitrate is NO3- nitrite is NO2- —Preceding unsigned comment added by 131.118.39.101 (talk) 22:39, 14 December 2007 (UTC)[reply]

Done. -lysdexia 16:00, 16 December 2007 (UTC) —Preceding unsigned comment added by 68.127.229.114 (talk)

Is this a reliable source of values? I've been doing some simulations on diamond - but I've not found anywhere else that lists a value as high as 10. The values on this clipper website don't appear to be referenced - so there is no clue as to their validity. Why not use eg. http://www.kayelaby.npl.co.uk/general_physics/2_6/2_6_5.html - which would be the standard text in universities. — Preceding unsigned comment added by Wideofthemark (talk • contribs) 11:05, 6 September 2013 (UTC) --Wideofthemark (talk) 11:09, 6 September 2013 (UTC)[reply]

At optical frequencies, it should be the square of the index of refraction, so about 5.8 for diamond. Not close to 10. Gah4 (talk) 02:40, 3 July 2021 (UTC)[reply]

Glass: Index of refraction and dielectric constant

Am I the only person who has noticed the discrepancy between the index of refraction n of glass (around 1.5 visible light), and the dielectric constant of glass, usually noted as being greater than 3.8. Since the dielectric constant (for nonmagnetic materials) is n², then one would expect to see a value of 1.5² = 2.25 Can anyone explain this to me? —The preceding unsigned comment was added by 155.92.180.59 (talk) 23:20, 16 March 2007 (UTC).[reply]

Your value only applies for frequencies in the visible range. The listed values are for a static electric field. In full, the dielectric constant is a complex, wavelength dependent 'constant'. -- Daan

That is definitely true for polar liquids that can rotate and align with the field. It is less obvious for glass, but some ions might be able to move. Gah4 (talk) 02:43, 3 July 2021 (UTC)[reply]

Nowadays, people use "dielectric constant" for the relative permittivity at any frequency

The article says that "dielectric constant" is only used for the static relative permittivity. Historically, I believe this was true, but in present usage it is commonly used for the frequency dependent relative permittivity. Just Google "dielectric constant frequency" if you don't believe me.

Because the definition in the article does not match present usage, I'm marking it with the disputed tag as I don't have a chance to fix it right now.

Arguably, the most unambiguous thing would be to have two pages, one on relative permittivity for the frequency-dependent relative permittivity tensor, and one on static relative permittivity for the electrostatic relative permittivity (what the current "dielectric constant" article is about). Both of them would mention "dielectric constant" as a common synonym, albeit a possibly ambiguous one.

—Steven G. Johnson 16:44, 8 August 2007 (UTC)[reply]

Okay, I've moved the page to have an unambiguous title, and clarified the situation of the term "dielectric constant" with a few references to demonstrate its ambiguity and obsolescence. It still feels like we should have a main page on relative permittivity with most of the definitions etc., with this page just as a smaller special case. But it's fine for now, I guess. —Steven G. Johnson 18:36, 9 August 2007 (UTC)[reply]

The page now says "Second, while in most modern usage "dielectric constant" refers to a relative permittivity[7][9], it may be either the static or the frequency-dependent relative permittivity depending on context." But, but ... the title of this page is 'relative permittivity', whilst the first line explicitly states it is only about 'static relative permittivity'. IMHO the usage with dielectric constant, whilst having some potential for confusion, is arguably better than what is said here about relative permittivity? This article implies that 'static relative permittivity' is always implied when 'relative permittivity' is encountered on its own, and that when talking about 'frequency-dependent relative permittivity' one must always refer to it in full. I don't think everyone follows those rules? Aarghdvaark (talk) 10:57, 19 April 2010 (UTC)[reply]

I noticed that despite strong language in this article differentiating between "dielectric constant" and "relative permittivity" and having dielectric constant link to that very statement, the confusion expressed here is still highly persistent throughout Wikipedia articles. Aside from that, dielectric constant is still very common jargon in the electric engineering industry, used interchangeably with relative permittivity. I even corrected two instances where the term was still 'improperly' used in this same article. If the chosen logic is to link dielectric constant to a simple statement of its ambiguity and obsolescence, then "relative permittivity" or "static relative permittivity" should be substituted anywhere that "dielectric constant" is presently used in any article for consistency. Perhaps it would actually be more correct if usage is ambiguous infrequently versus nearly every link for that term contradicting its usage? 66.195.220.180 (talk) 21:11, 8 June 2016 (UTC)[reply]

(La,Nb):(Zr,Ti)PbO3

In the tablle, there's standing:

(La,Nb):(Zr,Ti)PbO3	500,6000

what number should this be? 5,006,000 or 500,600 or 500.6000 or what? PLease fix! —Preceding unsigned comment added by 88.217.34.60 (talk) 14:04, 6 November 2007 (UTC)[reply]

Don't include original research

User:Brews ohare keeps trying to insert the following unsourced claim, which is original research:

Contrariwise, departure of a measured ε_r from 1 is evidence that the measured medium is not "vacuum".

The problem with this claim is that it implies that it is possible to measure the linear permittivity of a putative vacuum, which is not possible unless you have a reference vacuum to compare against. (This is equivalent to the impossibility of measuring the speed of light in a vacuum, which is impossible because the speed of light in vacuum defines the meter.)

Brews has been fighting on Vacuum permittivity to get his original claim included, see Talk:Vacuum permittivity, and has been inserting similar claims (or claims that imply his claims) on other pages such as this one. As has been pointed out on Talk:Vacuum permittivity, there are multiple references that state unequivocally that the linear relative permittivity of a vacuum is one by definition of the units, and is hence not a measurable quantity (like the speed of light in vacuum); Brews has been unable to provide any references to the contrary.

—Steven G. Johnson (talk) 14:52, 19 February 2008 (UTC)[reply]

PS. Please respond to this on [[Talk:Vacuum permittivity], so that the same discussion does not get repeated in multiple places. —Steven G. Johnson (talk) 14:54, 19 February 2008 (UTC)[reply]

I will respond further on Talk:Vacuum permittivity; however, I wish to note here that this is a misunderstanding of my position on the matter.Brews ohare (talk) 15:00, 19 February 2008 (UTC)[reply]

Rice Husk Ash

What is the dielectric constant of Rice Husk Ash? 210.187.49.65 (talk) 00:53, 3 March 2008 (UTC)[reply]

dielectric constant of earth or sand

what is the dielectric constant of a) dry earth (or sand)? b) wet earth (or sand)? Thunderbird2 (talk) 08:10, 8 May 2008 (UTC)[reply]

Lawrence B. Conyers' table at NADAG may be of interest. Hunterd is back! 09:45, 14 April 2010 (UTC)[reply]

Small mistake in the table of dielectric constants

Hey, I just wanted to point out that conjugated polymers have dielectric constant from 1.8 to 6 and only polymers with giant polarization have 100000. Just read a first page of the reference (nr. 3) provided...

Therefore I recommend correcting from "6-10000" to "1.8-6 up to 100000".

All the best. —Preceding unsigned comment added by Mrtransient (talk • contribs) 14:18, 7 May 2009 (UTC)[reply]

What if we split it into two subcategories: Into polymers and polymers with giant polarization. — Preceding unsigned comment added by 81.241.94.171 (talk) 06:38, 18 January 2019 (UTC)[reply]

Lossy Medium - constant k

There must be something wrong with the approximate value of the constant k in this paragraph, since k is not just a number but has units.--91.3.122.84 (talk) 19:24, 7 September 2009 (UTC)[reply]

Move Request

The following discussion is an archived discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. No further edits should be made to this section.

The result of the move request was page moved. Skomorokh 07:35, 27 December 2009 (UTC)[reply]

The article actually covers also the frequency dependence of ε_r. Therefore, "static" should be eliminated from the lemma. -- Marie Poise (talk) 18:17, 7 December 2009 (UTC)[reply]

The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page. No further edits should be made to this section.

Units not Listed

SI units for relative permittivity are not listed, and if it's dimensionless then this should also be stated. Preferably early in the article so as to be easy to find. I would do it myself, but I can't determine it with enough confidence. —Preceding unsigned comment added by ThorFreyaSaturn (talk • contribs) 02:53, 11 April 2010 (UTC)[reply]

Added to the lead. Relative permittivities are always dimensionless. Materialscientist (talk) 04:26, 11 April 2010 (UTC)[reply]

New Reference for Dielectric Constants Needed

Looks like reference #1 is a bad link, anyone have a better reference for Dielectric Constants of Materials? — Preceding unsigned comment added by Hebbster (talk • contribs) 13:38, 11 October 2011 (UTC)[reply]

There is a table archived by the University of Arkansas that is based on a table published by GSSI, a well-known manufacturer of GPR equipment: https://wayback.archive-it.org/6471/20130302062147/http://www.cast.uark.edu/nadag/EducationalMaterials/GPR/RDPtable.htm

Markaeologist (talk) 08:08, 10 July 2017 (UTC)[reply]

In: Chemical applications

Will this be right if added at the end of title section? Thanks in advance.

The commonly known "like-dissolves-like" principle could be useful here, as the probable reason for the discrepancy is the specific interaction between the oxygen atoms, as the THF could be treated as a homologue of water.

--Esmu Igors (talk) 19:15, 19 January 2012 (UTC)[reply]

DK?

I've seen reference to "DK" in circuit-board dielectrics. Is that this? This page doesn't mention DK. —Ben FrantzDale (talk) 18:56, 19 June 2012 (UTC)[reply]

Dielektrizitätskonstante? Materialscientist (talk) 22:49, 19 June 2012 (UTC)[reply]

DK is frequently used for dielectric constant/relative dielectric permittivity. Whether it is from German or not I don't know. Maybe more likely that 'DC' was considered too confusing an abbreviation when talking about electricity.

Markaeologist (talk) 08:27, 10 July 2017 (UTC)[reply]

Proposed merge of Permittivity#Measurement

Why is the measurement of permittivity more relevant in relative permittivity than it is in permittivity? On the face of it it seems to be a counterproductive move. Spinning Spark 12:12, 13 September 2013 (UTC)[reply]

Dielectric constant of metals

OK, I have also read in some university-level handbook that for metals ε = 1, as it is mentioned in this article. And this seems logical, as metals in an ideal case would not lessen electric field intensity in them as E = E₀. On the other hand, some good reference (i. e., Tomasi et al., Chem. Rev. 2005, 105, 2999−3093) claims that for an ideal conductor ε is equal to infinity, which also seems to make sense, if we look onto the table of ε for various materials provided in this article (more conducting materials have greater value of ε). Initially I had thought that it is only the electronic part of dielectric constant (the article mentioned is related to solvent models in quantum chemical calculations, particularily the claim about ε = ∞ is from the section about COSMO). But the vacuum has ε = 1, just the same as metals! How could that be possible? Is this related to the static/dynamic issue? Please clarify!

One thought I have is that, on the one hand, metals are excellent conductors of DC and low frequency AC, but, on the other hand, they do not permit an electric wave to enter them (as in Faraday's cage). I suspect the answer is somewhere in that direction, but I really do not have time to mine it... Are metals conducting only with their surface layer?! Please help. --Esmu Igors (talk) 19:54, 25 December 2013 (UTC)[reply]

The contradiction may be between the true dielectric constant and the effective dielectric constant. The true dielectric constant is determined by the dipoles formed by the fixed charges of the crystal lattice. The dipoles are very small hence (true) permittivity is close to that of free space and hence the relative permittivity is close to one. However, we also wish to take into account the effect of the free charges due to the material's conductivity. This results in effective permittivity of a complex number. The imaginary part is due to the conductivity and is given by the conductivity divided by the angular frequency. This number is usually very large in conductors and will completely swamp the true permittivity. An ideal conductor has infinite conductivity, and hence infinite (and imaginary) effective permittivity. Spinning Spark 22:01, 25 December 2013 (UTC)[reply]

Thank You for this clarifying. Though could You please desribe in short the relationship between the true ε_r and the effective ε_r? I wonder how imaginary part of ε_r can interact with the real part (in swamping)... Is effective ε_r the modulus of the complex value?

Another thing. As the imaginary part of ε_r is usually connected with absorption, might I then think that conductivity of metals arises from their "absorption capability" (absorbtivity) of charges injected from the rest of circuit? And what relates to dielectrics with high values of ε_r – are their high values somehow related to the imaginary part of ε_r? On the other hand, as far as I can judge, in these dielectrics permanent dipols could be quite large... --Esmu Igors (talk) 15:14, 26 December 2013 (UTC)[reply]

Lede accuracy

"...permittivity is the amount of energy stored in the material per unit voltage applied to the electrodes"

Really? The energy stored in a dielectric is proportional to V² just to start off with, and the constant of proportionality depends also on the form factor of the capacitor. I believe the correct relationship is given in terms of energy per unit volume and field strength as,

W=1/2\epsilon E^{2}

I suggest removing the sentence altogether as it is not summarising anything in the article (even incorrectly) so does not belong in the lede. Also, the word "similarly" at the beginning of the next sentence should go, that is not similar at all, and if we delete this sentence the compararand has gone. Spinning Spark 17:52, 9 June 2014 (UTC)[reply]

Small mistake in the table of dielectric constants(concrete dielectric)

The concrete dielectric is much too low (or these are measured at a frequency that isn't mentioned). The average dielectric for concrete in the 1000-3000MHz range is 8.5, which is our basis when starting a concrete scan with GPR. — Preceding unsigned comment added by 110.142.1.8 (talk) 04:20, 1 December 2014 (UTC)[reply]

Agreed that it's too low - Conyers & Goodman (1997) give an RDP of 6 for concrete; Baker et al. (2007) give a value of 7 for dry limestone (which may be regarded as somewhat analogous).

Baker, G.S., Jordan, T.E. & Pardy, J. 2007. ‘An introduction to ground penetrating radar (GPR)’. In Baker, G.S. & Jol, H.M. (eds.). Stratigraphic Analyses Using GPR (Special Paper 432) (1-18).

Baker, G.S. & Jol, H.M. (eds.). 2007. Stratigraphic Analyses Using GPR (Special Paper 432). Boulder, Colorado: The Geological Society of America.

Conyers, L.B. & Goodman, D. 1997. Ground-Penetrating Radar: an Introduction for Archaeologists. London & Walnut Creek CA: AltaMira/Sage.

Markaeologist (talk) 15:34, 6 March 2015 (UTC)[reply]

Permittivity less than one

@Eric Kvaalen: Regarding this edit, composite structures with an effective permittivity less than unity, or even negative, are known. See [2] for instance. Having said that, I agree that the lead is less confusing after your edit. SpinningSpark 18:04, 3 March 2015 (UTC)[reply]

Hello, there is NO dielectric 'permitivity' or however you call that, that is less than 1.000000.... as this constant is defined against vacuum!!!! (See Maxwells Equations).

Gerhard Oed www.oed_gd@web.de — Preceding unsigned comment added by 2003:66:8877:82C3:40E4:183A:D4CB:E5FF (talk) 03:41, 6 February 2016 (UTC)[reply]

Gerhard, did you even read the source I linked to? How about this one? SpinningSpark 09:40, 6 February 2016 (UTC)[reply]

Using metamaterials, it is certainly possible to obtain negative relative permittivity; this is well-established.Feydun (talk) 17:33, 14 February 2016 (UTC)[reply]

Both dielectric constant and index of refraction can go negative at specific frequencies. In physics terms, the phase velocity is allowed to be greater than c. The group velocity, at which information travels, has to stay less than c, which has some restrictions on the derivative(s) of either one. Gah4 (talk) 02:51, 3 July 2021 (UTC)[reply]

fact check the table of constants?

Folks, I'm worried about the table of constants, I am not sure thy are correct. Can we please take it down, and assemble it one citation at a time precisely specifying at what frequency range we're operating? --Frozenport (talk) 05:28, 16 November 2015 (UTC)[reply]

There is no need to take it down altogether, it can be worked on where it is. An additional column for frequency of measurement might be helpful, and perhaps another explicitly for references, which will make it clear what exactly is not referenced. By all means take out any unreferenced lines that seem dubious, but the table is much more likely to be worked on and improved where it is than it would be hidden on the talk page. SpinningSpark 14:33, 17 November 2015 (UTC)[reply]

For one, polarizable liquids, such as water, have very high dielectric constant at low frequencies, when the molecules can follow the field, but not so high when they can't. Many materials commonly used as dielectrics don't have this change. For such liquids, a low frequency (DC limit) and high frequency (microwave to optical) region would be useful. For finer detail, users should find a better reference, with more accurate frequency dependencies. Gah4 (talk) 01:39, 12 November 2018 (UTC)[reply]

Assessment comment

The comment(s) below were originally left at Talk:Relative permittivity/Comments, and are posted here for posterity. Following several discussions in past years, these subpages are now deprecated. The comments may be irrelevant or outdated; if so, please feel free to remove this section.

In the 'Practical relevance', the line "If a material with a high dielectric constant is placed in an electric field, the magnitude of that field will be measurably reduced within the volume of the dielectric." is confusing because a material with high permittivity (and thus dielectric constant) should increase the strength of the electric field between two metal plates and thus allow it to hold more charge. This contrasts to materials with smaller dielectric constants which reduce the electric fields. This is used in PCBs to insulate signal carrying conductors from each other. http://www.siliconfareast.com/dielectric-constant.htm (At the bottom). The section should be reworded to properly distinguish between the two applications given its dielectric constant value.

Last edited at 07:34, 27 December 2009 (UTC). Substituted at 04:07, 30 April 2016 (UTC)

Sources for table?

Anyone have sources for the dielectric constant of paper? I cannot find that value (k=3.85) anywhere else on the web. Jsmith7342 (talk) 11:47, 7 February 2017 (UTC)[reply]

Paper used to be a common dielectric for capacitors, so should be somewhat known, but otherwise, the composition of paper is variable enough that it won't be easy to find a specific value. Gah4 (talk) 01:40, 12 November 2018 (UTC)[reply]

First Google Books hit I saw came up with many specific values - you have to pick which kind of paper you mean, and the moisture content, density and frequency all affect it. Table 1 on page 348 of "Handbook of Physical Testing of Paper Vol. 2" (CRC Press, 2001 ISBN 0203910494,) says printing paper at 2.3% moisture content over a range of 0.1 to 200 kHz has a dielectric constant of 1.4. Lowest given is filter paper at 1.33, highest is 2.86 for cotton cellulose paper. Table 19 on page 4-30 of "Reference Data for Radio Engineers" says "royalgrey paper" has a constant of 3.30 at 60 Hz. "Standard Handbook For Electrical Engineers 11th Ed " ( which I am slowly retyping into the WP in its entirety) says on pages 4-173 and 4-174 that the constant for paper is a function of the density of the paper. Paper in HV cables has a constant around 2.25 at power frequency and moderate temperature. So, 3.85 sounds on the high side. We should probably show a more typical value and make a note of the wide range possible. --Wtshymanski (talk) 15:58, 14 November 2018 (UTC)[reply]

dielectric constant or relative permittivity?

There is discussion in coaxial cable on changing from dielectric constant to relative permittivity, so I came here to see what it says. Physics books that I know, old and new, all call it dielectric constant. I note references for the name change in engineering and chemistry, but not physics. I also note that the table is still labeled as dielectric constant. I might wonder about the WP:COMMONNAME? Gah4 (talk) 02:06, 31 October 2018 (UTC)[reply]

Compound with relative permittivity of a million missing

There is a compound with a relative permittivity of a million. This compound has a wikipedia page wherein it says that something in the year 2000. And I'm not sure but it could contain the element chromium. And be made by mixing multiple compounds together in air ar high temperature of between hundreds and thousands of degrees centigrade. Sadly I can't find the page back. 81.241.94.171 (talk) 06:42, 18 January 2019 (UTC)[reply]

List of permittivity page

Dear fellow Wikipedians. There needs to be a dedicated Wikipedia page that's a list of relative permittivities. A separate page that's just about this commonly used electrical property in engineering. And This page should then be referenced in the current Relative Permittivity page. Both in the see also section and with the short table of relative permittivities. 81.241.94.171 (talk) 06:47, 18 January 2019 (UTC)[reply]

I suppose so. Though there are a lot of materials that could go into such a list. Just about every compound in "The Handbook of Chemistry and Physics" could go in. (I don't remember now, does the handbook list it?). For many materials, there is a strong frequency dependence. Also, many materials (paper, concrete, glass) are variable enough that it is hard to find a single number. Gah4 (talk) 08:53, 18 January 2019 (UTC)[reply]

Coulomb force between two point charges

The article says Coulomb force between two point charges. Seems to me that it could be two line charges, or any other shape that one can calculate from theory. Gah4 (talk) 22:46, 5 December 2019 (UTC)[reply]

As for actually measuring it, it seems to be done using a disk^[1] with a guard ring, such that the field is perpendicular to the disk. There is then a correction for the gap. Gah4 (talk) 22:54, 5 December 2019 (UTC)[reply]

References

^ "The Kelvin Absolute Voltmeter" (PDF). www.sophphx.caltech.edu. Caltech. Retrieved 5 December 2019.

Confusing sentence

At the beginning of the article, it says: "Relative permittivity is the factor by which the electric field between the charges is decreased relative to vacuum." The higher the relative permittivity is, the higher the field will be in the medium. The sentence implies the opposite, as it compares the relative permittivity to a measure of decrease. Just as absorbance would be a decrease of light intensity. This sentence is extremely confusing. 129.69.48.110 (talk) 12:54, 13 July 2021 (UTC)SomeGuy[reply]

Data page

I propose to replace the table with a data table for common compounds' relative permittivity. Here are few sources that can be used as a starting point (please don't extract data manually): Google: Dielectric Constants pdf AXONOV (talk) ⚑ 08:34, 25 July 2022 (UTC)[reply]

Ice at microwave and optical frequencies

The dielectric constant (ok, relative permittivity) of water at frequencies low enough for the dipoles to follow is about 80. They can mostly follow up to microwave frequencies, which is how microwave ovens work. In ice, they can't follow so easily because of the crystal hydrogen bonds, so it should be less. Also, ice absorbs microwaves less well, complicating defrosting. The optical index of refraction of ice is about 1.31, so the dielectric constant at optical frequency, square of index of refraction, is about 1.7. Gah4 (talk) 04:04, 4 April 2023 (UTC)[reply]

high frequency

For polar liquids, low frequency means that the dipoles can follow the changing field, and high frequency means that they can't. I usually use "high frequency" to apply for optical frequencies, but others seem to have a different idea. In any case, ice should be closer to 1.7 at optical frequencies. It is always the square of the index of refraction, as long as that doesn't always mean optical frequencies. Gah4 (talk) 06:55, 19 April 2023 (UTC)[reply]

Good observation. The current source presumably is restricted to observations in a range of radio frequencies, and the permittivity of ice might be expected to change significantly below or in the microwave region. Perhaps we should find a way to qualify the wording: "low frequency" and "high frequency" are largely undefined here. —Quondum 11:55, 19 April 2023 (UTC)[reply]

Minimum value

Nowhere does the article answer the question of whether a material can have a relative permittivity lower than unity. Is a vacuum the limit? 74.73.228.182 (talk) 15:04, 9 December 2023 (UTC)[reply]

Relative permeability, and its square root, index of refraction, can go below one near a resonance. There are extra rules that restrict the shape of the curve, such that causality isn't violated. They can actually go below zero. Gah4 (talk) 23:23, 10 December 2023 (UTC)[reply]

[Panofsky-1] "The Kelvin Absolute Voltmeter" (PDF). www.sophphx.caltech.edu. Caltech. Retrieved 5 December 2019.

[1]