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*. |

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*. |

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*. |

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*. |

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*. |

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*. |

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*. |