The amount of coherence can readily be measured by the interference visibility, which looks at the size of the interference fringes relative to the input waves (as the phase offset is varied) a precise mathematical definition of the degree of coherence is given by means of correlation functions. Two waves are said to be coherent if they have a constant relative phase. When interfering, two waves can add together to create a wave of greater amplitude than either one (constructive interference) or subtract from each other to create a wave of lesser amplitude than either one (destructive interference), depending on their relative phase. Constructive or destructive interference are limit cases, and two waves always interfere, even if the result of the addition is complicated or not remarkable. A single wave can interfere with itself, but this is still an addition of two waves (see Young's slits experiment). Interference is the addition, in the mathematical sense, of wave functions. More generally, coherence describes all properties of the correlation between physical quantities of a single wave, or between several waves or wave packets. It contains several distinct concepts, which are limiting cases that never quite occur in reality but allow an understanding of the physics of waves, and has become a very important concept in quantum physics. Coherence is an ideal property of waves that enables stationary (i.e., temporally or spatially constant) interference. In physics, two wave sources are coherent if their frequency and waveform are identical. Equivalence relation on waves with equal frequencies and waveforms
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