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Interferometry is the technique of using the pattern of interference created by the superposition of two or more waves to diagnose the properties of the aforementioned waves. In physics interference is the addition ( superposition) of two or more Waves that result in a new wave pattern In Physics and Systems theory, the superposition principle, also known as superposition property, states that for all Linear systems A wave is a disturbance that propagates through Space and Time, usually with transference of Energy. The instrument used to interfere the waves together is called an interferometer. Interferometry is an important investigative technique in the fields of astronomy, fiber optics, engineering metrology, optical metrology, oceanography, seismology, quantum mechanics and plasma physics. Astronomy (from the Greek words astron (ἄστρον "star" and nomos (νόμος "law" is the scientific study An optical fiber (or fibre) is a Glass or Plastic fiber that carries Light along its length Metrology (from Ancient Greek metron (measure and logos (study of is the Science of Measurement. Oceanography (from the greek words Ωκεανός meaning Ocean and γράφω meaning to write also called oceanology or Quantum mechanics is the study of mechanical systems whose dimensions are close to the Atomic scale such as Molecules Atoms Electrons In Physics and Chemistry, plasma is an Ionized Gas, in which a certain proportion of Electrons are free rather than being bound

Contents

Basic Principle

See also: Interference
The light path through a Michelson interferometer.
The light path through a Michelson interferometer. In physics interference is the addition ( superposition) of two or more Waves that result in a new wave pattern Michelson interferometer is the most common configuration for optical Interferometry and was invented by Albert Abraham Michelson.

Interferometry makes use of the principal of superposition to combine separate waves together in a way that will cause the result of their combination to have some meaningful property that is diagnostic of the original state of the waves. This works because when two waves with the same frequency combine the resulting pattern is determined by the phase difference between the two waves -- waves that are in phase will undergo constructive inference while waves that are out of phase will undergo destructive interference. Frequency is a measure of the number of occurrences of a repeating event per unit Time. The phase of an oscillation or wave is the fraction of a complete cycle corresponding to an offset in the displacement from a specified reference point at time t = 0 Most interferometers use light or some other form of electromagnetic wave. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter.

Typically a single incoming beam of light will be split into two identical beams by a grating or a partial mirror. Each of these beams will travel a different route, called a path, before they are recombined at a detector. The path difference, the difference in the distance travelled by each beam, creates a phase difference between them. It is this introduced phase difference that creates the interference pattern between the initially identical waves. If a single beam has been split along two paths then the phase difference is diagnostic of anything that changes the phase along the paths. This could be a physical change in the path length itself or a change in the refractive index along the path. In Chemistry, the path length is defined as the distance that light ( UV / VIS) travels through a sample in an analytical cell The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium

Imaging Interferometry

The pattern of radiation across a region can be represented as a function of position i(x,y), i. e. an image. The pattern of incoming radiation i(x,y) can be transformed into the fourier domain f(u,v). This article specifically discusses Fourier transformation of functions on the Real line; for other kinds of Fourier transformation see Fourier analysis and A single detector measures information from a single point in x,y space. An interferometer measures the difference in phase between two points in the x,y domain. This corresponds to a single point in the u,v domain. The signals from each set of detectors is combined in a device called a correlator. A single detector builds up a full image by scanning through the x,y coordinates. An interferometry builds up a full picture by measuring multiple points in u,v space. The image i(x,y) can then be restored by preforming a fourier transform on the measured f(u,v) data. This technique is called aperture synthesis.

Applications

Astronomical Interferometry

For more details on this topic, see Astronomical interferometer. An astronomical interferometer is an array of telescopes or mirror segments acting together to probe structures with higher resolution
The VLA interferometry.
The VLA interferometry. The Very Large Array ( VLA) is a Radio astronomy Observatory located on the Plains of San Augustin, between the towns of Magdalena

The angular resolution that a telescope can achieve is determined by its diffraction limit (which is proportional to its diameter). Angular resolution describes the resolving power of any image forming device such as an optical or Radio telescope, a Microscope, a Camera A telescope is an instrument designed for the observation of remote objects and the collection of Electromagnetic radiation. The resolution of an optical imaging system like a Microscope or Telescope or Camera can be limited by multiple factors like imperfections in the lenses or misalignment The larger the telescope, the better its resolution. However, the cost of building a telescope also scales with its size. The purpose of astronomical interferometry is to achieve high-resolution observations using a cost-effective cluster of comparatively small telescopes rather than a single very expensive monolithic telescope. The basic unit of an astronomical interferometry is a pair of telescopes. Each pair of telescopes is a basic interferometer. Their position in u,v space is referred to as a baseline.

Early astronomical interferometry was involved with a single baseline being used to measure the amount of power on a particular small angular scale. See also Astronomical interferometer William Herschel knew as early as 1779 (Herschel 1805 that Stars appeared much larger in telescopes than they Later astronomical interferometers were telescope arrays comprised of a set of, usually identical, telescopes arranged in a pattern on the ground. A limited number of baselines will result in insufficient coverage in u,v space. This can be alleviated by using the rotation of the earth to rotate the array relative to the sky. This causes the points in u,v space that each baseline points at to change with time. Thus, a single baseline can measure information along a track in u,v space just by taking repeated measurements. This technique is called earth-rotation synthesis. It is even possible to have baseline of tens, hundreds, or even thousands of kilometers by using a technique called very long baseline interferometry. Very Long Baseline Interferometry (VLBI is a type of astronomical interferometry used in Radio astronomy.

The longer the wavelength of incoming radiation the easier it is to measure its phase information. For this reason early imaging interferometry was almost exclusively done with long wavelength radio telescopes. Examples of radio interferometers include the VLA and MERLIN. The Very Large Array ( VLA) is a Radio astronomy Observatory located on the Plains of San Augustin, between the towns of Magdalena The Multi-Element Radio Linked Interferometer Network ( MERLIN) is an Interferometer array of Radio telescopes spread across England and the As the speed of correlators and associated technologies have improved the wavelength of radiation usable with interferometry has decreased. There have been several submillimeter inferometers with the largest, the Atacama Large Millimeter Array, currently under construction. The Atacama Large Millimeter/submillimeter Array ( ALMA) is an international Astronomy project that consists of an Astronomical interferometer formed Optical astronomical interferometers have traditionally been specialised instruments, but recent developments have broadened their capabilities. Current Performance of Ground-Based Interferometers Here is a list of currently existing astronomical optical interferometers (i

Optical Interferometry


See also

References

Dictionary

interferometry

-noun

  1. (physics) the design and use of optical or radio interferometers
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