# An Etymological Dictionary of Astronomy and AstrophysicsEnglish-French-Persian

## فرهنگ ریشه شناختی اخترشناسی-اخترفیزیک

### M. Heydari-Malayeri    -    Paris Observatory

Homepage

Number of Results: 8 Search : coherence
 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 hamdusiFr.: 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: Ac = D2λ2/(πd2), 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 hamdusiFr.: 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 hamdusiFr.: 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 hamdusiFr.: 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 = (Imax - Imin) / (Imax + Imin), where Imax is the intensity at a maximum of the → interference pattern, and Imin 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 kuantomiFr.: 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 fazâyiFr.: 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 zamâniFr.: 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 lc = λ2/(2πΔλ); lc is called the → coherence length.→ temporal; → coherence.