Editing Waveguide (section)

== Electromagnetic waveguides ==

=== Radio-frequency waveguides ===
{{Main|Waveguide (radio frequency)|Transmission line}}
Waveguides can be constructed to carry waves over a wide portion of the [[electromagnetic spectrum]], but are especially useful in the [[microwave]] and [[optics|optical]] frequency ranges. Depending on the frequency, they can be constructed from either [[Electrical conduction|conductive]] or [[dielectric]] materials. Waveguides are used for transferring both [[power (physics)|power]] and communication signals.{{sfn|Cronin|1995}}{{rp|1–3}}{{sfn|Okamoto|2010}}{{rp|xiii-xiv}}

[[File:Radar waveguide.JPG|thumb|In this military radar, microwave radiation is transmitted between the source and the reflector by a waveguide. The figure suggests that microwaves leave the box in a circularly symmetric mode (allowing the antenna to rotate), then they are converted to a linear mode, and pass through a flexible stage. Their polarisation is then rotated in a twisted stage and finally they irradiate the parabolic antenna.]]

=== Optical waveguides ===
{{Main|Waveguide (optics)}}
Waveguides used at optical frequencies are typically dielectric waveguides, structures in which a [[dielectric]] material with high [[permittivity]], and thus high [[index of refraction]], is surrounded by a material with lower permittivity. The structure guides optical waves by [[total internal reflection]]. An example of an optical waveguide is [[optical fiber]].{{sfn|Herres}}

Other types of optical waveguide are also used, including [[photonic-crystal fiber]], which guides waves by any of several distinct mechanisms. Guides in the form of a hollow tube with a highly reflective inner surface have also been used as [[light pipe]]s for illumination applications. The inner surfaces may be polished metal, or may be covered with a multilayer film that guides light by [[Bragg reflection]] (this is a special case of a photonic-crystal fiber).  One can also use small [[prism (optics)|prism]]s around the pipe which reflect light via total internal reflection<ref>{{cite web | url=http://www.physics.ubc.ca/ssp/research/lightpipe.htm | archive-url=https://web.archive.org/web/20070314173338/http://www.physics.ubc.ca/ssp/research/lightpipe.htm | archive-date=March 14, 2007 | title=Light Pipe Technologies }}</ref> —such confinement is necessarily imperfect, however, since total internal reflection can never truly guide light within a ''lower''-index core (in the prism case, some light leaks out at the prism corners).{{sfn|Saxe|1989}}