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A retroreflector reflects incoming light and directs it back to its source. The light is reflected parallel to the direction it came from, but in the completely opposite direction. Common types include the corner cube retroreflector, which can send light back long distances and is useful on highways. Other common types are mirrors, paints, tapes, and retroreflective arrays, such as those that have been placed on the moon or satellites. Retroreflection can be accomplished with corner shaped or spherical elements as well as with phase conjugation, an optical phenomenon that can accurately direct light in lasers and optical transmission lines.
Corner retroreflectors are most commonly made of transparent optical glass. The light is reflected either completely by internal optics or by a coating on the outer surface. Another kind of this type of retroreflector has perpendicular mirrors that border an open space. These corner reflectors are sometimes used on roadways, although spherical reflectors cause light to return at a slightly different angle. This prevents bright headlights of other vehicles from shining in a driver’s eyes and blinding them.
There are many road related applications for a retroreflector. Such devices can be embedded, so that they are even with the road surface, or installed as raised devices. Ones that are raised are rarely seen in places where it snows, because plows would tear them out. Retroreflective paint does not cause wear of the road surface to accelerate, like embedded devices do, although the paint gets worn away by weather and vehicles passing over it.
On several moon missions, a retroreflector array was placed aboard equipment to track lunar rotation and position as well as to disprove theories that such landings were hoaxes. Such devices were used on some Apollo as well as Russian missions. The laser ranging retroreflector was, and still is, used for this purpose, and can be tracked periodically using earth-based telescopes.
Retroreflector products are also used on boats as well as life rafts and other floatation systems. There are also retroreflectors used on clothing for certain photography applications, and others that are used for surveying and for disabling digital cameras. The glowing effect of eyes is an example of retroreflection, as the reflection of light from the lens, eye fluid, and behind the retina causes an effect that any retroreflector provides, regardless of its type.
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