The Interference reference article from the English Wikipedia on 24-Apr-2004
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Interference

See the real Africa

In communications, interference is anything which alters, modifies, or disrupts a message; as it travels along a channel, between a source and a receiver.


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

Interference of
two circular waves

image:wavepanel.png
Wavelength
(decreasing bottom to top)
and
Wave centers distance
(increasing to the right)
.
Absolute value snapshots of the
(real-valued, scalar) wave field.
As time progresses, the wave
fronts would move outwards
from the two centers,
but the dark regions
(destructive interference) stay fixed.

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 energy 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 "coherent," and have all of their photons' phases aligned with each other. Lasers 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 Sun) 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.

Constructive and Destructive interference

When two 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 amplitude between 0 and 2A depending on if the two waves are in phase or out of phase.

combined
waveform
Image:Interference of two waves.png
wave 1
wave 2

Two waves in phaseTwo waves 180° out
of phase

Consider two waves that are in phase,with amplitudes A1 and A2. Their troughs and humps line up and and the resultant wave will have amplitude A = A1+A2. This is known as constructive interference.

If the two wave's are 180° out of phase, then one waves hump will coincide with another wave's trough and so will tend to cancel out. The resultant amplitude A = |A1 - A2|. If A1=A2 the resultant amplitude will be zero. This is known as destructive interference.

See also moiré pattern, interferometer, beat.