coherence همدوسی hamdusi (#)
*Fr.: cohérence*
The property of two or more electromagnetic waves when they are in
fixed phase relationship over time. If the crests and troughs of the waves
meet at the same time and place they are said to be in phase. → *cohere* + *-ence*, → *-ance*. |

coherence area پهنهی ِ همدوسی pahne-ye hamdusi
*Fr.: zone de cohérence*
Of an → *electromagnetic wave*,
the area of a surface perpendicular to the direction of
→ *propagation*, over which the
wave maintains a specified
→ *degree of coherence*.
According to the van Cittert-Zernike theorem, the
coherence area is given by:
*A*_{c} =
*D*^{2}λ^{2}/(π*d*^{2}),
where *d* is the diameter of the light source and
*D* is the distance away.
The coherence area is an important parameter in photon correlation
experiments. In the → *Young's experiment*
the → *interference pattern* is only seen if
slits are inside one coherence area. → *coherence*; → *area*. |

coherence length درازای ِ همدوسی derâzâ-ye hamdusi
*Fr.: longueur de cohérence*
The distance over which an → *electromagnetic wave*
train maintains a specified
→ *degree of coherence*.
The coherence length is related to the
→ *coherence time* multiplied by
vacuum → *velocity of light*. → *coherence*; → *length*. |

coherence time زمان ِ همدوسی zamân hamdusi
*Fr.: temps de cohérence*
The time over which a propagating → *electromagnetic wave*
may be considered → *coherent*.
The coherence time of an interferometer
is the interval during which the fringe phase remains stable. → *coherence*; → *time*. |

degree of coherence درجهی ِ همدوسی dareje-ye hamdusi
*Fr.: degré de cohérence*
The extent of → *coherence*
of an → *electromagnetic wave*,
as indicated by a → *dimensionless* number.
Since interference takes place when waves are
→ *coherent*,
using a → *Young's experiment*,
the degree of coherence
is measured from the → *fringe*
→ *visibility*, *V*.
It is defined as the ratio
*V* = (*I*_{max} - *I*_{min}) /
(*I*_{max} + *I*_{min}),
where *I*_{max} is the intensity at a maximum of
the → *interference pattern*,
and *I*_{min}
is the intensity at a minimum of
the interference pattern.
The electromagnetic wave is considered to be highly coherent when the degree of
coherence is about 1, incoherent for nearly zero values, and
partially coherent for values between 0 and 1. → *degree*; → *coherence*. |

quantum coherence همدوسی ِ کوآنتومی hamdusi-ye kuantomi
*Fr.: cohérence quantique*
In quantum physics, a situation where an object's wave
property is split in two, and the two waves coherently interfere
with each other in such a way as to form a single state
that is a superposition of the two states.
This phenomenon is based on the fact that atomic particles have
wave-like properties.
Quantum coherence is in many ways similar to
→ *quantum entanglement*, which involves the
shared states of two quantum particles instead of two quantum waves
of a single particle.
Quantum coherence and quantum entanglement are both rooted in the
→ *superposition principle*. → *quantum*; → *coherence*. |

spatial coherence همدوسی ِ فضایی hamdusi-ye fazâyi
*Fr.: cohérence spatiale*
In a wave train, a correlation between the phases of waves at points separated in space at a given
time. → *spatial*; → *coherence*. |

temporal coherence همدوسی ِ زمانی hamdusi-ye zamâni
*Fr.: cohérence temporelle*
A measure of the correlation between the phases of an
→ *electromagnetic wave*
at different points
along the direction of propagation. Temporal coherence indicates
to what extent a source is monochromatic. Imagine a
source emitting waves with wavelength λ ± Δλ.
Waves with wavelength
λ and λ + Δλ, which at some point in space constructively
interfere, will no longer
constructively interfere after some path length
*l*_{c} = λ^{2}/(2πΔλ); *l*_{c}
is called the → *coherence length*. → *temporal*; → *coherence*. |