First-order Temporal Coherence in Classical Optics

A truly monochromatic beam of light, if it ever existed, would be perfectly coherent. Such a beam could be split into two parts and each part propagated over an arbitrary distance. When the parts were finally brought together and mixed, no matter how different the two path lengths may have been, the resulting waveform would exhibit constructive and destructive interference in the form of bright and dark fringes. The coherence length of a monochromatic beam is therefore infinite, in the sense that the path length difference can be as large as desired, without hampering one's ability to create interference patterns.

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