An Etymological Dictionary of Astronomy and Astrophysics
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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.

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 = 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.

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.

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.

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.

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.

In a wave train, a correlation between the phases of waves at points separated in space at a given time.

→ spatial; → coherence.

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 = λ²/(2πΔλ); l_c is called the → coherence length.

→ temporal; → coherence.