Editing Josiah Willard Gibbs (section)

=== Physical optics ===
[[File:Calcite.jpg|right|thumb|alt=Photograph showing birefrigence by a calcite crystal|A [[calcite]] crystal produces birefringence (or "double refraction") of light, a phenomenon which Gibbs explained using Maxwell's equations for electromagnetic phenomena.]]

Though Gibbs's research on physical optics is less well known today than his other work, it made a significant contribution to classical [[electromagnetism]] by applying [[Maxwell's equations]] to the theory of optical processes such as [[birefringence]], [[Dispersion (optics)|dispersion]], and [[Optical rotation|optical activity]].<ref name="Bumstead" /><ref name="Optics">Wheeler 1998, ch. VIII</ref>  In that work, Gibbs showed that those processes could be accounted for by Maxwell's equations without any special assumptions about the microscopic structure of matter or about the nature of the medium in which electromagnetic waves were supposed to propagate (the so-called [[Luminiferous aether|luminiferous ether]]).  Gibbs also stressed that the absence of a [[longitudinal wave|longitudinal]] electromagnetic wave, which is needed to account for the observed properties of [[light]], is automatically guaranteed by Maxwell's equations (by virtue of what is now called their "[[Gauge fixing|gauge invariance]]"), whereas in mechanical theories of light, such as Lord Kelvin's, it must be imposed as an ''ad hoc'' condition on the properties of the aether.<ref name="Optics" />

In his last paper on physical optics, Gibbs concluded that

{{blockquote|it may be said for the electrical theory [of light] that it is not obliged to invent hypotheses, but only to apply the laws furnished by the science of electricity, and that it is difficult to account for the coincidences between the electrical and optical properties of media unless we regard the motions of light as electrical.|J. W. Gibbs, 1889<ref name="Bumstead" />}}

Shortly afterwards, the electromagnetic nature of light was demonstrated by the experiments of [[Heinrich Hertz]] in Germany.<ref name="Hertz">{{cite book | last = Buchwald | first = Jed Z. | author-link = Jed Buchwald |title = The Creation of Scientific Effects: Heinrich Hertz and Electric Waves |publisher = University of Chicago Press |year = 1994 |isbn = 978-0-226-07887-8}}</ref>