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How does coherence in laser light allow interference patterns to be recorded?

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hologram graphic — holography Figure 1: Gabor's original method for creating holograms.

Coherence is essential to holography because it ensures that light waves maintain a constant phase relationship, allowing for the formation of stable interference patterns.12

In the recording process, a laser beam is split into two parts: an object beam that reflects off the subject and a reference beam that travels directly to the recording medium. Because the light is coherent, these two beams arrive at the photographic plate with a fixed phase relationship. When they overlap, they interfere:342
* Constructive interference occurs where the waves are in phase (crests align with crests), creating bright areas.34
* Destructive interference occurs where the waves are out of phase (crests align with troughs), creating dark areas.34

This interaction creates a complex pattern of interference fringes that encodes both the intensity and the phase information of the light reflected from the object. Unlike ordinary photography, which only records intensity, this stored phase information allows for the reconstruction of a three-dimensional representation of the original object.14