Editing Wave interference (section)

=== Interferometry ===
{{Main|Interferometry}}
Interferometry has played an important role in the advancement of physics, and also has a wide range of applications in physical and engineering measurement. The impact on physics and the applications span various types of waves.

==== Optical interferometry ====
{{Main|Optical interferometry}}

[[Thomas Young (scientist)|Thomas Young]]'s double slit interferometer in 1803 demonstrated interference fringes when two small holes were illuminated by light from another small hole which was illuminated by sunlight.  Young was able to estimate the wavelength of different colours in the spectrum from the spacing of the fringes.  The experiment played a major role in the general acceptance of the wave theory of light.<ref name="Born and Wolf" />
In quantum mechanics, this experiment is considered to demonstrate the inseparability of the wave and particle natures of light and other quantum particles ([[wave–particle duality]]). [[Richard Feynman]] was fond of saying that all of quantum mechanics can be gleaned from carefully thinking through the implications of this single experiment.<ref name="Greene_1999">{{cite book|last =Greene|first =Brian|author-link =Brian Greene|title =The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory|publisher =W.W. Norton|location =New York|year =1999|pages =[https://archive.org/details/elegantuniverses0000gree/page/97 97–109]|isbn =978-0-393-04688-5|title-link =The Elegant Universe: Superstrings, Hidden Dimensions, and the Quest for the Ultimate Theory}}</ref>

The results of the [[Michelson–Morley experiment]] are generally considered to be the first strong evidence against the theory of a [[luminiferous aether]] and in favor of [[special relativity]].

Interferometry has been used in defining and calibrating [[Length measurement|length standards]].  When the metre was defined as the distance between two marks on a platinum-iridium bar, [[Albert Abraham Michelson|Michelson]] and Benoît used interferometry to measure the wavelength of the red [[cadmium]] line in the new standard, and also showed that it could be used as a length standard. Sixty years later, in 1960, the metre in the new [[SI]] system was defined to be equal to 1,650,763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.  This definition was replaced in 1983 by defining the metre as the distance travelled by light in vacuum during a specific time interval. Interferometry is still fundamental in establishing the [[calibration]] chain in length measurement.

Interferometry is used in the calibration of [[Gauge block|slip gauges]] (called gauge blocks in the US) and in [[coordinate-measuring machine]]s. It is also used in the testing of optical components.<ref>RS Longhurst,  ''Geometrical and Physical Optics'', 1968, Longmans, London.</ref>

==== Radio interferometry ====
{{Main|Astronomical interferometer}}

[[File:USA.NM.VeryLargeArray.02.jpg|thumb|The [[Very Large Array]], an [[interferometer|interferometric array]] formed from many smaller [[telescope]]s, like many larger [[radio telescope]]s.]]
In 1946, a technique called [[Astronomical interferometer|astronomical interferometry]] was developed. Astronomical radio interferometers usually consist either of arrays of parabolic dishes or two-dimensional arrays of omni-directional antennas. All of the telescopes in the array are widely separated and are usually connected together using [[coaxial cable]], [[waveguide]], [[optical fiber]], or other type of [[transmission line]]. Interferometry increases the total signal collected, but its primary purpose is to vastly increase the resolution through a process called [[Aperture synthesis]]. This technique works by superposing (interfering) the signal waves from the different telescopes on the principle that waves that coincide with the same phase will add to each other while two waves that have opposite phases will cancel each other out. This creates a combined telescope that is equivalent in resolution (though not in sensitivity) to a single antenna whose diameter is equal to the spacing of the antennas farthest apart in the array.

==== Acoustic interferometry ====
An [[acoustic interferometer]] is an instrument for measuring the physical characteristics of sound waves in a [[gas]] or liquid, such [[velocity]], wavelength, [[absorption (acoustics)|absorption]], or [[Electrical impedance|impedance]]. A vibrating [[crystal]] creates ultrasonic waves that are radiated into the medium. The waves strike a reflector placed  parallel to the crystal, reflected back to the source and measured.