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Parallel waves coming in to a parabolic mirror are focused at a point F

A parabolic reflector (or dish or mirror) is a parabola-shaped reflective device, used to collect or distribute energy such as light, sound, or radio waves.

Contents

Theory

The parabolic reflector functions due to the geometric properties of the paraboloid shape: if the angle of incidence to the inner surface of the collector equals the angle of reflection, then any incoming ray that is parallel to the axis of the dish will be reflected to a central point, or "focus". Because many types of energy can be reflected in this way, parabolic reflectors can be used to collect and concentrate energy entering the reflector at a particular angle. Similarly, energy radiating from the "focus" to the dish can be transmitted outward in a beam that is parallel to the axis of the dish. Parabolic reflectors suffer from an aberration called coma. This is primarily of interest in telescopes because most other applications do not require sharp resolution off the axis of the parabola.

History

Parabolic mirrors were first described by the Arabian physicist, Ibn Sahl, in the 10th century1. James Gregory, in his 1663 book Optica Promota (1663), pointed out that a reflecting telescope with a mirror that was parabolic would correct spherical aberration as well as the chromatic aberration seen in refracting telescopes. The design he came up with bears his name: the "Gregorian telescope"; but according to his own confession, Gregory had no practical skill and he could find no optician capable of actually constructing one2. Isaac Newton knew about the properties of parabolic mirrors but chose a spherical shape for his Newtonian telescope mirror to simplify construction3. John Hadley introduced parabolic mirrors into practical astronomy in 1721 when he used one in his reflecting telescope4. Lighthouses also commonly used parabolic mirrors to collimate a point of light from a lantern into a beam, before being replaced by more efficient fresnel lenses in the 19th century.

Applications

The most common modern applications of the parabolic reflector are in satellite dishes, reflecting telescopes, radio telescopes, parabolic microphones, and many lighting devices such as spotlights, car headlights, PAR Cans and LED housings.

The Olympic Flame has been lit using a parabolic reflector concentrating sunlight. Parabolic mirrors are one of many shapes for a burning-glass.

A parabolic reflector pointing upward can be formed by rotating a reflective liquid, like mercury, around a vertical axis. This makes the liquid mirror telescope possible.

Parabolic mirror wok with optical illusion

A mirage bowl or parabolic mirror wok is a toy consisting of two parabolic mirrors that are put together like a wok pan and a lid with a hole in it. The device produces upright real image (with the magnification 1) of an object placed on the bottom of it, which gives the optical illusion that something is in the hole of the lid (while actually it is at the bottom of the pan below).

Parabolic reflectors are a popular alternative for increasing wireless signal strength. Even with simple ones, users have reported 3 dB or more gains.[1]

References

  1. ^ The Forgotten Revolution By Lucio Russo, Silvio Levy Page 331
  2. ^ A Biographical Dictionary of Eminent Scotsmen By Robert Chambers, Thomas - Page 175
  3. ^ Electronic Imaging in Astronomy By Ian S. McLean - Page 77
  4. ^ Electronic Imaging in Astronomy By Ian S. McLean - Page 77

See also

External links
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