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Why do interference fringes have alternating bright and dark areas?
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holography
Figure 1: Gabor's original method for creating holograms.
In the context of holography, interference fringes consist of alternating bright and dark areas due to the superposition of two coherent light beams—typically a reference beam and a beam reflected from an object.1
The formation of these bands is governed by the phase relationship between the two waves when they meet on a recording medium, such as a photographic plate:1
- Bright areas occur through constructive interference, where the two beams are in phase. In this state, the waves reinforce each other as crest meets crest and trough meets trough.12
- Dark areas occur through destructive interference, where the two waves are of equal amplitude but opposite phase. Here, the crest of one wave meets the trough of the other, resulting in complete cancellation.12
This pattern of fringes effectively records the phase and amplitude information of the light reflected from an object, which is essential for reconstructing a three-dimensional image in holography.1