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Here is my understanding of how holograms work: A holographic plate captures the interference pattern between a reference beam and an object beam.

When reconstructing the hologram, the reference beam is incident on the holographic plate which reproduces the object beam (and some other beams that we don't see??) on the side of the reference beam which allows us to view a 3D (virtual) image of the object.

I don't understand how the reference beam would reproduce the object beam when it interacts with the plate. I would like to know exactly what happens at the point when the reference beam hits the holographic plate (when reconstructing the image). I'm looking for a non-mathematical explanation if possible.

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  • $\begingroup$ One word hint: "diffraction". $\endgroup$ – The Photon Mar 17 '18 at 16:11
  • $\begingroup$ @ThePhoton but why does it diffract in such a way that we only see the object beam? $\endgroup$ – zld123 Mar 18 '18 at 11:32
  • $\begingroup$ Because it was created by the interaction of the reference beam with a beam reflected from the original object. $\endgroup$ – The Photon Mar 18 '18 at 15:56
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Non-mathematical? I'll give it a shot.

When the object beam and reference beam intersect on the plate, they modify the optical properties of the recording medium that is on the plate: either they cause the optical absorption to change, or they cause the refractive index to change, according to the intensity of the light at each point. Interference between the object and reference beams produces a pattern of light and less-light fringes on the plate, so the recording medium acquires a corresponding pattern of dark/light fringes or high/low refractive index fringe.

Denis Gabor earned his Nobel prize for realizing that the fringe patterns thus recorded amount to diffraction gratings that, upon illumination with either one of the original two beams, will reconstruct the other beam.

As for why the recorded fringes diffract light precisely that way, it's much easier to explain in mathematical terms, but it can also be explained via Huygens principle. If you look at Figure 2 on this page, you will see how the wavelength of light, the direction of illumination, and the spacing of lines in a diffraction grating can control the direction into which incident light is diffracted. What Gabor realized is that the diffraction grating that produces a given diffracted beam from a given incident beam is precisely the diffraction grating (i.e., fringe pattern) produced by two beams identical to the incident beam and the diffracted beam.

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