# Dictionary Definition

interference

### Noun

1 a policy of intervening in the affairs of other
countries [syn: intervention] [ant:
nonintervention,
nonintervention]

2 the act of hindering or obstructing or impeding
[syn: hindrance]

3 electrical or acoustic activity that can
disturb communication [syn: noise, disturbance]

4 (American football) the act of obstructing
someone's path with your body; "he threw a rolling block into the
line backer" [syn: blocking, block]

5 any obstruction that impedes or is burdensome
[syn: hindrance,
hitch, preventive, preventative, encumbrance, incumbrance]

# User Contributed Dictionary

## English

### Pronunciation

- ɪntɹˌˈfiɹɨns

### Noun

- The act of interfering with something, or something that interferes.
- The illegal obstruction of an opponent in some ball games.
- They were glued to the TV, as the referee called out a fifteen yard penalty for interference.

- An effect caused by
the superposition
of two systems of
waves, such as a distortion on a broadcast signal due to atmospheric or other
effects.
- They wanted to watch the game on TV, but there was too much interference to even make out the score on the tiny screen.

#### Translations

the act of interfering with something

- Finnish: häirintä, väliintulo, puuttuminen
- Swedish: interferens

the illegal obstruction of an opponent in some
ball games

- Finnish: häirintä

an effect caused by the superposition of two
systems of waves

- Finnish: interferenssi
- German: Interferenz

a distortion on a broadcast signal due to
atmospheric or other effects

- Finnish: häiriö

### Derived terms

# Extensive Definition

In physics, interference is the addition
(superposition)
of two or more waves that
result in a new wave pattern.

As most commonly used, the term interference
usually refers to the interaction of waves which are correlated or
coherent
with each other, either because they come from the same source or
because they have the same or nearly the same frequency.

Two non-monochromatic waves are
only fully coherent
with each other if they both have exactly the same range of
wavelengths and the
same phase
differences at each of the constituent wavelengths.

The total phase difference is derived from the
sum of both the path difference and the initial phase difference
(if the waves are generated from 2 or more different sources). It
can then be concluded whether the waves reaching a point are in
phase (constructive interference) or out of phase (destructive
interference).

## Theory

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

## Experiments

Thomas
Young's double-slit
experiment showed interference phenomena where two beams of
light which are coherent interfere to produce a pattern.

The beams of light both have the same wavelength
range and at the center of the interference pattern. They have the
same phases at
each wavelength, as they both come from the same source.

## Interference patterns

For two coherent sources, the spatial separation between sources is half the wavelength times the number of nodal lines.Light from any source can be used to obtain
interference patterns, for example, Newton's
rings can be produced with sunlight. However, in general
white light is less suited
for producing clear interference patterns, as it is a mix of a full
spectrum of colours, that each have different spacing of the
interference fringes. Sodium light
is close to monochromatic and is thus
more suitable for producing interference patterns. The most
suitable is laser light
because it is almost perfectly monochromatic.

## Constructive and destructive interference

Consider two waves that are in phase,with
amplitudes A_1 and A_2. Their troughs and peaks line up and the
resultant wave will have amplitude A = A_1 + A_2. This is known as
constructive interference.

If the two waves are π radians, or 180°, out of phase,
then one wave's crests will coincide with another wave's troughs
and so will tend to cancel out. The resultant amplitude is A = |A_1
- A_2|. If A_1 = A_2, the resultant amplitude will be zero. This is
known as destructive interference.

When two sinusoidal waves superimpose, the
resulting waveform depends on the frequency (or wavelength)
amplitude and relative phase of the two waves. If the two waves
have the same amplitude A and wavelength the resultant waveform
will have an amplitude between 0 and 2 A depending on whether the
two waves are in phase or out of
phase.

## General quantum interference

If a system is in state \psi its wavefunction is described in Dirac or bra-ket notation as:- |\psi \rang = \sum_i |i\rang \psi_i

where the |i\rangs specify the different quantum
"alternatives" available (technically, they form an eigenvector basis)
and the \psi_i are the probability
amplitude coefficients, which are complex
numbers.

The probability of observing the system making a
transition or quantum leap
from state \Psi to a new state \Phi is the square of the modulus of
the scalar or
inner
product of the two states:

prob(\psi \Rightarrow \phi) = |\lang \psi |\phi
\rang|^2 = |\sum_i\psi^*_i \phi_i |^2 = \sum_ \psi^*_i \psi_j
\phi^*_j\phi_i= \sum_ |\psi_i|^2|\phi_i|^2 + \sum_ \psi^*_i \psi_j
\phi^*_j\phi_i

where \psi_i = \lang i|\psi \rang (as defined
above) and similarly \phi_i = \lang i|\phi \rang are the
coefficients of the final state of the system. * is the complex
conjugate so that \psi_i^* = \lang \psi|i \rang , etc.

Now let's consider the situation classically and
imagine that the system transited from |\psi \rang to |\phi \rang
via an intermediate state |i\rang. Then we would classically expect
the probability of the two-step transition to be the sum of all the
possible intermediate steps. So we would have

prob(\psi \Rightarrow \phi) = \sum_i prob(\psi
\Rightarrow i \Rightarrow \phi) = \sum_i |\lang \psi |i
\rang|^2|\lang i|\phi \rang|^2 = \sum_i|\psi_i|^2 |\phi_i|^2.

The classical and quantum derivations for the
transition probability differ by the presence, in the quantum case,
of the extra terms \sum_ \psi^*_i \psi_j \phi^*_j\phi_i; these
extra quantum terms represent interference between the different i
\ne j intermediate "alternatives". These are consequently known as
the quantum interference terms, or cross terms. This is a purely
quantum effect and is a consequence of the non-additivity of the
probabilities of quantum alternatives.

The interference terms vanish, via the mechanism
of quantum
decoherence, if the intermediate state |i\rang is measured or
coupled with the environment.

## Examples

A conceptually simple case of interference is a
small (compared to wavelength) source - say, a small array of
regularly spaced small sources (see diffraction
grating).

Consider the case of a flat boundary (say,
between two media with different densities or simply a flat
mirror), onto which the plane wave is incident at some angle. In
this case of continuous distribution of sources, constructive
interference will only be in specular direction - the
direction at which angle with the normal is exactly the same as the
angle of incidence. Thus, this results in the law of
reflection which is simply the result of constructive
interference of a plane wave on a plane surface.

## See also

## References

## External links

interference in Min Nan: Kan-sia̍p

interference in Bosnian: Interferencija
talasa

interference in Czech: Interference

interference in Danish: Interferens

interference in German: Interferenz
(Physik)

interference in Estonian: Interferents

interference in Spanish: Interferencia

interference in French: Interférence

interference in Galician: Interferencia

interference in Korean: 간섭

interference in Croatian: Interferencija
valova

interference in Ido: Interfero

interference in Indonesian: Interferensi

interference in Italian: Interferenza
(fisica)

interference in Hebrew: התאבכות

interference in Lithuanian: Interferencija

interference in Hungarian: Interferencia

interference in Dutch: Interferentie
(natuurkunde)

interference in Japanese: 干渉 (物理学)

interference in Norwegian: Interferens

interference in Polish: Interferencja

interference in Portuguese: Interferência

interference in Romanian: Interferenţă

interference in Russian: Интерференция
(физика)

interference in Slovak: Interferencia
(vlny)

interference in Slovenian: Interferenca

interference in Serbian: Интерференција

interference in Finnish: Interferenssi

interference in Vietnamese: Giao thoa

interference in Turkish: Girişim

interference in Ukrainian: Інтерференція

interference in Chinese: 波的干涉

# Synonyms, Antonyms and Related Words

amplitude, antagonism, antinode, antipathy, arrest, arrestation, arrestment, atmospherics, backlash, blaring, blasting, blind spot, block, blockage, blocking, check, clashing, clogging, closing up, closure, collision, conflict, confutation, constriction, contradiction, contraposition, contrariety, counteraction, counterposition,
counterworking,
cramp, crankiness, crawling, creeping, crest, crotchetiness, de Broglie
wave, delay, detainment, detention, difficulty, diffraction, dissent, drift, electromagnetic radiation,
electromagnetic wave, encroachment, encumbrance, entrance, entrenchment, fade-out,
fading, fixation, foot-dragging,
frequency, frequency
band, frequency spectrum, friction, frustration, guided wave,
hampering, handicap, hindering, hindrance, hitch, holdback, holdup, impedance, impediment, impingement, imposition, in phase,
incursion, infiltration, influx, infringement, inhibition, injection, inroad, insinuation, interceding, intercession, interjection, interloping, interposition, interposure, interruption, intervention, intruding, intrusion, invasion, irruption, kick, let, light, longitudinal wave,
mechanical wave, meddling, negativism, node, noise, nonconformity, nuisance
value, obstruction,
obstructionism,
obtrusion, occlusion, opposition, opposure, oppugnance, oppugnancy, out of phase,
period, periodic wave,
perverseness, radio
wave, ray, reaction, recalcitrance, reception, recoil, reinforcement, renitency, repercussion, repression, repugnance, resistance, resonance, resonance
frequency, restraint,
restriction,
retardation,
retardment, revolt, seismic wave, setback, shock wave, snag, sound wave, squeeze, static, stranglehold, stricture, suppression, surface wave,
swimming upstream, tidal wave, transverse wave, trespass, trespassing, trough, unlawful entry, wave, wave equation, wave motion,
wave number, wavelength