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For example, the flaw in the Hubble Space Telescope mirror (too flat by about 2,200 nm at its perimeter) caused severe spherical aberration until corrected with COSTAR. For comparison, the diameter of a human hair is usually about 50,000 nm, so the required accuracy for a reflector to focus visible light is about 2500 times less than the diameter of a hair. The wavelength range of visible light is between about 400 and 700 nanometres (nm), so in order to focus all visible light well, a reflector must be correct to within about 20 nm. To prevent this, the dish must be made correctly to within about 1 / 20 of a wavelength. If the dish is wrong by a quarter of a wavelength, then the reflected energy will be wrong by a half wavelength, which means that it will interfere destructively with energy that has been reflected properly from another part of the dish. The precision to which a parabolic dish must be made in order to focus energy well depends on the wavelength of the energy. This is primarily of interest in telescopes because most other applications do not require sharp resolution off the axis of the parabola.
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However, if the incoming beam makes a non-zero angle with the axis (or if the emitting point source is not placed in the focus), parabolic reflectors suffer from an aberration called coma.
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In contrast with spherical reflectors, which suffer from a spherical aberration that becomes stronger as the ratio of the beam diameter to the focal distance becomes larger, parabolic reflectors can be made to accommodate beams of any width. 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. (For a geometrical proof, click here.) 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. The parabolic reflector functions due to the geometric properties of the paraboloidal shape: any incoming ray that is parallel to the axis of the dish will be reflected to a central point, or " focus". For off-axis reflectors (with just the part of the paraboloid between the points P 1 and P 3), the receiver is still placed at the focus of the paraboloid, but it does not cast a shadow onto the reflector. The vertex is V, and the axis of symmetry passes through V and F. Parallel rays coming into a parabolic mirror are focused at a point F. If a parabola is positioned in Cartesian coordinates with its vertex at the origin and its axis of symmetry along the y-axis, so the parabola opens upward, its equation is 4 f y = x 2 are defined as above. (The distinction is like that between a sphere and a circle.) However, in informal language, the word parabola and its associated adjective parabolic are often used in place of paraboloid and paraboloidal. A parabola is the two-dimensional figure. Strictly, the three-dimensional shape of the reflector is called a paraboloid. ( November 2012) ( Learn how and when to remove this template message) Unsourced material may be challenged and removed. Please help improve this section by adding citations to reliable sources. In acoustics, parabolic microphones are used to record faraway sounds such as bird calls, in sports reporting, and to eavesdrop on private conversations in espionage and law enforcement. In radio, parabolic antennas are used to radiate a narrow beam of radio waves for point-to-point communications in satellite dishes and microwave relay stations, and to locate aircraft, ships, and vehicles in radar sets. In optics, parabolic mirrors are used to gather light in reflecting telescopes and solar furnaces, and project a beam of light in flashlights, searchlights, stage spotlights, and car headlights. Since the principles of reflection are reversible, parabolic reflectors can also be used to collimate radiation from an isotropic source into a parallel beam. Parabolic reflectors are used to collect energy from a distant source (for example sound waves or incoming star light). Conversely, a spherical wave generated by a point source placed in the focus is reflected into a plane wave propagating as a collimated beam along the axis. The parabolic reflector transforms an incoming plane wave travelling along the axis into a spherical wave converging toward the focus. Its shape is part of a circular paraboloid, that is, the surface generated by a parabola revolving around its axis.
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One of the world's largest solar parabolic dishes at the Ben-Gurion National Solar Energy Center in IsraelĪ parabolic (or paraboloid or paraboloidal) reflector (or dish or mirror) is a reflective surface used to collect or project energy such as light, sound, or radio waves.