How does constructive interference produce bright regions on a hologram?

In holography, bright regions on a hologram are produced through the process of interference. When the light beam reflected from an object and a separate coherent reference beam meet on a photographic plate, they combine their wave amplitudes to create a pattern of interference fringes.1

Constructive interference occurs when these two beams are in phase—meaning the crest of one wave meets the crest of the other, and the trough meets the trough. Because the waves are "in step," they reinforce each other, resulting in a bright area on the recording medium. Conversely, when the waves are of equal amplitude but opposite phase (crest meeting trough), they cancel each other out to produce a dark area. This complex pattern of light and dark fringes effectively records the phase and amplitude information of the light reflected from the object, which allows for the later reconstruction of a three-dimensional image.12