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How does destructive interference produce dark regions on a hologram?
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holography
Figure 1: Gabor's original method for creating holograms.
In holography, dark regions on a hologram are produced through the process of destructive interference when two coherent light beams—the reference beam and the beam reflected from the object—interact on the photographic plate.1
When these two waves meet, their amplitudes combine to form an interference pattern consisting of light and dark fringes. A dark area results when the two waves are of equal amplitude but are in opposite phase; specifically, this occurs when the crest of one wave meets the trough of the other, and the trough of the first meets the crest of the second. In this state, the waves cancel each other out, resulting in a region of complete destructive interference.12