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How do object and reference beams form interference fringes on a holographic plate?

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

In holography, the object and reference beams form interference fringes on a holographic plate through the interaction of two coherent light waves.12

When a laser beam illuminates an object, the light is reflected, scattered, and diffracted by the object's physical features, carrying both amplitude and phase information to the photographic plate. Simultaneously, a portion of the same laser beam is split off to serve as a reference beam, which is directed onto the same plate.12

As these two beams meet at the plate, they interfere with each other, creating a complex pattern of light and dark areas known as interference fringes. This occurs because:1
* Constructive interference: When the waves are in phase (crest meets crest, trough meets trough), they reinforce each other, creating light areas.1
* Destructive interference: When the waves are of equal amplitude but opposite phase (crest meets trough), they cancel each other, creating dark areas.1

This resulting pattern of fine, close-packed fringes records the complete optical field, allowing for the later reconstruction of a three-dimensional image.12