What is a hologram and how does it work

The Short Explanation
Holograms are made by exposing a piece of film to laser light, which is then scattered by the object being holographed. The film is also exposed to light coming directly from the laser (the reference beam). The two beams of light interfere when they reach the film because they have taken different paths and are no longer in phase with each other. The film simply records this interference pattern, which is the hologram. To reconstruct (view) the image, the hologram is illuminated by a beam of light, which is diffracted by the interference pattern on the film. This reproduces the original surface pattern of the object in three dimensions.

Holograms are truly 3D. Certain holograms permit one to walk all the way around the projected image and see it from above and below, as if the image were real. The other remarkable property of holograms is that one can cut away a small piece of the holographic film, hold it up to laser light, and still see an entire, intact, 3D image of the photographed object.

A hologram when viewed under the illumination of incoherent light such as the light from an incandescent light bulb reveals nothing. The observer sees only a smoky haze, the result of the laser-produced interference pattern.

If the holographic film is viewed with illumination from a source of coherent laser light, it reproduces the reference beam that helped to create the original interference pattern, and the object being photographed is revealed with all it's 3D characteristics.

If a small piece of that holographic film is now cut a way and examined under the illumination of laser light, a smaller, yet intact, original object can be seen. The reason for this is the fact that the hologram is an energy interference pattern. Within this pattern, every piece contains the whole. That is, if one could take a hologram of an apple, cut the film into fifty pieces, then each piece when viewed through the laser light would reveal it's own miniature apples.