Editing Corner reflector

{{short description|Retroreflector with three orthogonal, intersecting flat surfaces}}
{{distinguish|Corner reflector antenna}}
[[File:corner reflector.JPG|thumb|A corner reflector for radar testing]]
A '''corner reflector''' is a [[retroreflector]] consisting of three mutually [[perpendicular]], [[intersection (Euclidean geometry)|intersecting]] flat reflective surfaces.  It reflects [[wave]]s incident from any direction directly towards the source, but [[Translation (geometry)|translated]].   The three intersecting surfaces often are triangles (forming a [[Trirectangular tetrahedron|tetrahedron]]) or may have square shapes. Radar corner reflectors made of metal are used to reflect radio waves from [[radar]] sets. Optical corner reflectors, called '''corner cubes''' or '''cube corners''', made of three-sided glass [[Prism (optics)|prism]]s, are used in [[surveying]] and [[laser rangefinder|laser ranging]].

==Principle==
[[File:Corner reflector.svg|thumb|220px|Working principle of a corner reflector]]

The incoming ray is reflected three times, once by each surface, which results in a reversal of direction.<ref name="Newman">{{cite book   
  | last = Newman
  | first = William I. 
  | title = Continuum Mechanics in the Earth Sciences
  | publisher = Cambridge University Press
  | date = 2019
  | pages = 6–7
  | url = https://books.google.com/books?id=CWXO_nVRKYMC&pg=PA6
  | isbn = 978-0-521-56289-8}}</ref><ref name="Bernstein">{{cite book   
  | last1 = Bernstein
  | first1 = Matt A.
  |first2=William A. |last2=Friedman
  | title = Thinking About Equations: A Practical Guide for Developing Mathematical Intuition in the Physical Sciences and Engineering
  | publisher = Wiley
  | date = 2011
  | pages = 193
  | url = https://books.google.com/books?id=l8A-VnolePcC&dq=%22corner+reflector&pg=PT193
  | isbn = 978-1-118-21064-2}}</ref>   To see this, the three corresponding normal vectors of the corner's perpendicular sides can be considered to form a [[basis (linear algebra)|basis]] (a [[rectangular coordinate system]]) (''x'', ''y'', ''z'') in which to represent the direction of an arbitrary incoming ray, {{thinsp|[''a'',|''b'',|''c'']}}.  When the ray reflects from the first side, say ''x'', the ray's ''x'' component, ''a'',  is reversed to −''a'' while the ''y'' and ''z'' components are unchanged, resulting in a direction of {{thinsp|[−''a'',|''b'',|''c'']}}.  Similarly, when reflected from side ''y'' and finally from side ''z'', the ''b'' and ''c'' components are reversed. Therefore, the ray direction goes from {{thinsp|[''a'',|''b'',|''c'']}} to {{thinsp|[−''a'',|''b'',|''c'']}} to {{thinsp|[−''a'',|−''b'',|''c'']}} to {{thinsp|[−''a'',|−''b'',|−''c'']}}, and it leaves the corner reflector with all three components of direction exactly reversed. The distance travelled, relative to a plane normal to the direction of the rays, is also equal for any ray entering the reflector, regardless of the location where it first reflects.{{Citation needed|date=March 2013}}
[[File:Reflection-coin-cube-rot.gif|thumb|Animation showing the reflected rays in a corner of a cube (corner reflector principle).]]
{{Clear}}

==In radar==
{{multiple image
| align = center
| direction = horizontal
| header   = Corner reflectors used in radar
| image1   = Radarreflektor auf einer Motoryacht 3596.JPG
| caption1 = Octahedral corner reflector on the mast of a yacht. 
| width1   = 150
| image2   = buoy seal.jpg
| caption2 = Buoy in San Diego Harbor. Metal plates near the top form corner reflectors to reflect radar signals
| width2   = 148
| image3 = Radar reflector at Alte Brücke (Frankfurt am Main).jpg
| caption3 = Radar reflector on an [[abutment]] of a bridge
| width3 = 300 
| image4   = NTS - Reflector Tower.jpg
| caption4 = A multireflector at the [[Nevada Test Site]] used as radar target for simulated nuclear bombing
| width4   = 176
| footer   = <!-- Ack! {{notelist}} doesn't seem to work here, so approximate -->Note: The diamond-shaped corner reflector on the yacht is improperly deployed; to best reflect surface radar it should be deployed in the so-called "rain-catching" configuration so as to present an inside corner as shown on the "radar testing" image. 
}}

Radar corner reflectors are designed to reflect the [[microwave]] [[radio waves]] emitted by [[radar]] sets back toward the radar antenna.  This causes them to show a strong "return" on radar screens.  A simple corner reflector consists of three conducting sheet metal or screen surfaces at 90° angles to each other, attached to one another at the edges, forming a "corner".  These reflect radio waves coming from in front of them back parallel to the incoming beam.  To create a corner reflector that will reflect radar waves coming from any direction, 8 corner reflectors are placed back-to-back in an [[octahedron]] (diamond) shape.  The reflecting surfaces must be larger than several [[wavelength]]s of the radio waves to function.<ref>{{cite book|last1=Kraus |first1=John |last2=Marhefka |first2=Ronald |title=Antennas for All Applications |edition=3rd |date=2002|publisher=McGraw Hill|isbn=0-07-112240-0|page=365}}</ref>

In maritime navigation they are placed on [[bridge]] abutments, [[buoy]]s, [[ship]]s and, especially, [[Lifeboat (shipboard)|lifeboats]], to ensure that these show up strongly on ship radar screens.  Corner reflectors are placed on the vessel's masts at a height of at least {{convert|4.6|m|ft|abbr=in}} above sea level (giving them an approximate minimum [[horizon]] distance of {{convert|8|km|nmi|round=0.5|lk=out|abbr=off|sp=us|disp=or}}).  [[Marine radar]] uses [[X-band]] microwaves with wavelengths of {{convert|2.5|-|3.75|cm|in|round=0.5|abbr=in}}, so small reflectors less than {{convert|30|cm|in|abbr=in}} across are used.  In aircraft navigation, corner reflectors are installed on rural [[runway]]s, to make them show up on aircraft radar.

An object that has multiple reflections from smooth surfaces produces a radar return of greater magnitude than might be expected from the physical size of the object. This effect was put to use on the [[ADM-20 Quail]], a small decoy missile which had the same [[radar cross section]] as a [[B-52]].

The corner reflector is not the only efficient radar reflector design; other [[retroreflector]] designs have also seen use. [[Luneburg lens]], for example, are used on the [[ADM-141 TALD]].<ref>{{cite web |title=IMI ADM-141 TALD |url=https://www.designation-systems.net/dusrm/m-141.html |website=www.designation-systems.net}}</ref>

==In optics==
[[File:Corner Cube Reflector CCR.jpg|thumb|Corner cube reflector]]
[[Image:ALSEP AS15-85-11468.jpg|thumb|Apollo 15 Lunar Laser Ranging RetroReflector (LRRR) installed on the Moon]]

In [[optics]], corner reflectors typically consist of three [[mirror]]s or reflective [[prism (optics)|prism]] faces which return an incident [[light beam]] in the opposite direction. In [[surveying]], [[retroreflector]] prisms are commonly used as targets for long-range electronic distance measurement using a [[total station]].

Five arrays of optical corner reflectors have been placed on the [[Moon]] for use by [[Lunar Laser Ranging experiment]]s observing a [[laser]]'s [[time-of-flight]] to measure the Moon's orbit more precisely than was possible before.  The three largest were placed by [[NASA]] as part of the [[Apollo program]], and the [[Soviet Union]] built two smaller ones into the [[Lunokhod programme|Lunokhod rovers]].

[[Automobile]] and [[bicycle]] tail lights are molded with arrays of small corner reflectors, with different sections oriented for viewing from different angles.  Reflective [[paint]] for visibility at night usually contains [[retroreflector|retroreflective spherical beads]].
Thin plastic with microscopic corner reflector structures can be used as [[Flagging (tape)|tape]], on signs, or sewn or molded onto [[clothing]].

==Other examples==
Corner reflectors can also occur accidentally. [[Tower block]]s with [[balcony|balconies]] are often accidental [[acoustics|acoustic]] corner reflectors and return a distinctive [[echo]] to an observer making a sharp sound noise, such as a hand clap, nearby.

==See also==
*{{annotated link|Cat's eye (road)|Cat's eye reflector}}
*{{annotated link|LAGEOS}}
*{{annotated link|Lunar Laser Ranging experiment}}
*{{annotated link|Stealth technology}}

==References==
{{Reflist}}
*{{FS1037C MS188}}
*{{DODDIC}}

{{DEFAULTSORT:Corner Reflector}}
[[Category:Mirrors]]
[[Category:Radar]]