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.
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: 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.
Fr.: longueur de cohérence
Fr.: temps de cohérence
degree of coherence
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 = (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.
Fr.: cohérence spatiale
In a wave train, a correlation between the phases of waves at points separated in space at a given time.
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 lc = λ2/(2πΔλ); lc is called the → coherence length.