In a holography set-up, as shown in the figure below, Illumination beam and reference beam both are in phase. The interference pattern generated at the detector contains the whole information about the object. When illumination beam will incident on to the object its phase and amplitude will change and further object beam will recombines with reference beam at the detector. Question is how the Interference pattern provides complete 3D information of the object ?
I would perhaps answer something like this. Maybe that question helps you also. You have to keep in your mind the proper meaning of whole information; the plate will just contain almost the same information as the one you get from the other side of a window placed there (thorugh it), being its size the same of the plate's one, and provided you are working with the same light source (monochromatic).
Almost comes also from the fact that when reconstructing the stored image you have to know the shape of part of the light field used during the recording process (reference beam). That's what allows you to decode that information.
