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Below is an article about the physical phenomenon of wave interference. See also the legal concept of humanitarian interference, or interference in baseball.

Interference is the superposition (overlapping) of two or more waves resulting in a new wave pattern. As most commonly used, the term usually refers to the interference of waves of the same or nearly the same frequency.

The principle of superposition of waves states that the resultant displacement at a point is equal to the sum of the displacements of different waves at that point. If a crest of a wave meets a crest of another wave at the same point then the crests interefere constructively and the resultant wave amplitude is greater. If a crest of a wave meets a trough then they interfere destructively, and the overall amplitude is decreased.

Interference is involved in Thomas Young's double-slit experiment where coherent light (light which is in the same phase, and has the same frequency and wavelength) interferes to produce an interference pattern. More generally, this form of interference can occur whenever a wave can propagate from a source to a destination by two or more paths of different length. Two or more sources can only be used to produce interference when there is a fixed phase relation between them, but in this case the interference generated is the same as with a single source; see Huygens' principle. When a single source interferes with itself, the principle of conservation of energyConservation of energy the first law of thermodynamics is one of several conservation laws. It states that the total inflow of energy into a system must equal the total outflow of energy from the system, plus the change in the energy contained within the dictates that the energy "missing" from the darkened regions of an interference pattern where destructive interference has taken place will be found in the brightened portions where constructive interference has taken place.

Light beams that can produce interference patterns are called " coherentCoherence is a property of waves that measures the ability of the waves to interfere with each other. Two waves that are coherent can be combined to produce an unmoving distribution of constructive and destructive interference (a visible interference patt," and have all of their photons' phases aligned with each other. LaserFor alternative meanings see laser (disambiguation). A laser light amplification by stimulated emission of radiation is a device which uses a quantum mechanical effect, stimulated emission, to generate a coherent beam of light. Light from a laser is oftens are an example of a coherent light source. Light beams which cannot produce interference patterns are called "incoherent." Most ordinary incandescent light sources (including the SunThe Sun (also called Sol is the star in our solar system. Planet Earth orbits the Sun. Other bodies that orbit the Sun include other planets, asteroids, meteoroids, comets and dust. Not all objects passing through the solar system have been orbitally capt) are incoherent.

In a signal transmission system, "interference" refers to either extraneous power which tends to interfere with the reception of the desired signals or to the disturbance of the signals which results.