Editing Refractive index (section)

===Negative refractive index===
{{See also|Negative index metamaterials}}
[[File:Split-ring resonator array 10K sq nm.jpg|thumb|250px|alt=A 3D grid of open copper rings made from interlocking standing sheets of fiberglass circuit boards|A [[split-ring resonator]] array arranged to produce a negative index of refraction for [[microwaves]]]]

Recent research has also demonstrated the "existence" of materials with a negative refractive index, which can occur if [[permittivity]] and [[magnetic permeability|permeability]] have simultaneous negative values.<ref name=veselago1968>{{cite journal |last= Veselago |first= V. G. |author-link= Victor Veselago |title= The electrodynamics of substances with simultaneously negative values of ε and μ |journal= [[Physics-Uspekhi|Soviet Physics Uspekhi]] |date= 1968 |volume= 10 |issue= 4 |pages= 509–514 |doi= 10.1070/PU2003v046n07ABEH001614 |bibcode= 1968SvPhU..10..509V |s2cid= 250862458 }}</ref> This can be achieved with periodically constructed [[metamaterials]]. The resulting [[negative refraction]] (i.e., a reversal of [[Snell's law]]) offers the possibility of the [[superlens]] and other new phenomena to be actively developed by means of [[negative index metamaterials|metamaterials]].<ref>{{cite web |last1= Pendry |first1= J. B. |last2= Schurig |first2= D. |last3= Smith |first3= D. R. |title= Electromagnetic compression apparatus, methods and systems |url= https://patents.google.com/patent/US7629941 |id= US Patent 7629941 |date= Dec 8, 2009 |publisher= US Patent Office |via= Google Patents}}</ref><ref name=shalaev2007>{{cite journal |last= Shalaev |first= V. M. |author-link= Vladimir Shalaev |title= Optical negative-index metamaterials |journal= [[Nature Photonics]] |volume= 1 |issue= 1 |pages= 41–48 |date= 2007 |doi= 10.1038/nphoton.2006.49 |bibcode= 2007NaPho...1...41S |s2cid= 170678 }}</ref>