Corner cube

A corner reflector is a retroreflector consisting of three mutually perpendicular, intersecting flat surfaces, which reflects waves back directly towards the source, but translated. The three intersecting surfaces often have square shapes. Radar corner reflectors made of metal are used to reflect radio waves from radar sets. Optical corner reflectors, called corner cubes, made of three-sided glass prisms, are used in surveying and laser rangefinding.

The incoming ray is reflected three times, once by each surface, which results in a reversal of direction. To see this, the three corresponding normal vectors of the corner's perpendicular sides can be considered to form a basis (a rectangular coordinate system) (x, y, z) in which to represent the direction of an arbitrary incoming ray, [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 [−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 [a, b, c] to [−a, b, c] to [−a, −b, c] to [−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.

In diagram 1 the green ray is shown as reflecting from only one surface. This is a special case where the incoming ray is exactly normal (perpendicular) to one of the reflective faces. In the same diagram the orange and red rays are shown reflecting off of two surfaces. This is again a special case, the requirement being that the incoming ray is parallel to one of the reflecting planes. (In diagram 2, showing a retroreflective sphere, the incoming and outgoing rays are approximately parallel in special cases.)

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 wavelengths of the radio waves to function.

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Wikipedia