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coherent light \co*her"ent light\, n. (Physics, Optics)
Light in which the phases of all electromagnetic waves at
each point on a line normal to the direction of the the beam
are identical. Coherent light is usually monochromatic, and
the most common source of such light for practical uses is
from a laser.

Note: The constant phase relations of coherent light allows
the achievement of certain effects, such as holography,
not possible with noncoherent light. The low degree of
angular dispersion of coherent light beams also allows
the use of such light in laser ranging, over distances
as far as from the earth to the moon.
[PJC]