The refractive index of the plate is negative (
). Materials with negative refractive index do not exist in nature, but there are attempts to produce composite materials with this property.
With the help of the Demonstration, it can be seen that for
, a flat plate can focus light rays, and that if
, the image of the point source
is a point.
The Demonstration is also available as a web app on the Firefox Marketplace from
 V. Veselago, "The Electrodynamics of Substances with Simultaneously Negative Values of
," Soviet Physics Uspekhi
(4), 1968 pp. 509–514.
 G. V. Eleftheriades and K. G. Balmain, Negative-Refraction Metamaterials,
Hoboken, NJ: John Wiley & Sons, 2005.