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I'm not much familiar with holograms and I've read a few articles on how holograms are made. I have a question that how does hologram get projected in air? If it were tyndall effect, then we could see all of the path through which light goes but not the "holographic image" of the object? How is a light particle made to understand when it has to appear and when it has to disappear when it is passing through air? Help.

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A hologram is a two dimensional surface (or a volume) that can modulate both the brightness and the directionality of the diffracted light in every point. Looking at a hologram you perceive the objects "floating" in space, but the light is still diffracted at the surface or in the volume of the hologram.

In contrast a 2d image is only capable of modulating the brightness of the light. It has to look the same from every direction (except for perspective). A hologram, on the other hand, can produce one image when looked at from one direction and a different one from another. If the produced sequence of images that one can see during an eye movement around the hologram are that of a rotating object, then we will see a 3d representation of that object. That's a rather poor use of a hologram, by the way. One can make holograms that produce a completely different image for even slight rotations, and they would look like rapid turning of pages in a book. This is how one can use holograms for information storage.

That a hologram is not a free space projection is obvious from the fact that no hologram can ever project a picture outside of the rays between your eyes and its visible contour. Holograms can't do what is shown in the famous Leia hologram of the original Star Wars movies. The consequence of that is that small holograms can only show small images, whereas a small projector can project on a large surface. That, too, is not possible with holograms. If we wanted a hologram the size of a movie screen, then the entire screen would have to be the hologram.

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  • $\begingroup$ qz.com/438880/… $\endgroup$ – Quark Jan 9 '16 at 19:46
  • $\begingroup$ Check the video on this link , please. Why does it say "Display Medium : Air"? $\endgroup$ – Quark Jan 9 '16 at 19:47
  • $\begingroup$ @Quark: That is not a hologram but a 3d gas discharge display. Given the working principle I would be kind of afraid for my eyes. If you pump the air out of the display volume, the image will disappear. A hologram doesn't need the air and will continue to work. $\endgroup$ – CuriousOne Jan 9 '16 at 19:48
  • $\begingroup$ I just realised that too, that it is not actually a hologram. Why does plasma remain fixed to a position? $\endgroup$ – Quark Jan 9 '16 at 20:02
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    $\begingroup$ @Quark: Yes, they are using ultrashort laser pulses which produce a very strong electric field which accelerates electrons and ions in the air until a macroscopic gas discharge forms. One can probably greatly enhance the effect by pre-ionizing the working gas with UV or a silent gas discharge that adds free electrons to the gas. $\endgroup$ – CuriousOne Jan 9 '16 at 20:14
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To project a hologram into the air, you would need something in the air to scatter the light to your eyes. Otherwise you wouldn't see the hologram. This can be done by having some scattering particles in the air, like fog or dust.

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