HackerHow are holograms possible?
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Further reading 1
Caption notes
Film records intensity during exposure, rather than optical phase directly. Adding a coherent reference wave makes relative phase affect the interference pattern, enabling reconstruction with the reference.
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The exposure takes some time, and it's influenced by the average intensity of this wave over that time.When the first beam is in sync with that reference wave, the two will constructively interfere, producing a wave that has twice the amplitude, so the film gets exposed a lot at that point.