A hologram is very much like a photograph, they both are recordings of light. Nonetheless, several distinct differences in the way light is recorded make them worlds apart. Unlike photographs, holograms are not recordings of simply intensities of light, but also recordings of its phase distribution. In order to do that, holograms must be recorded with coherent light. Not until the laser was invented was it possible to create a hologram.
When light waves hit an object, it is reflected, scattered, and refracted by the surface of the object, changing the phase of the waves. Different objects of different shapes will change the form of the waves differently, leaving a distinct fingerprint, or impression, of the object onto the light waves. Now, if someone tried to take a picture of a bullet, assuming that it is taken at the right moment, the image of the bullet will most likely come out as a blurry streak. Taking a picture of a light wave, which happens to be quite a bit faster than a bullet, will be hundreds of times harder, if not impossible. Luckily, there is another way. When two waves come into contact, they interfere with each other, the resulting interference pattern is called a standing wave. As its name implies, as long as the two light waves do not change, the standing wave will remain the same and "stand still." This is what is recorded onto a hologram.
With that in mind, a hologram is created by first splitting a laser beam into two. One is called the object beam and the other is called the reference beam. The object beam is shined at the object, which is then reflected and exposed to the film. The reference beam is exposed to the film without any change. At the film, the reference beam interferes with the object beam, creating an interference pattern, a standing wave, which is recorded onto the film.
When a hologram is viewed with a coherent light source, the original light waves are reconstructed, creating a 3D image in space. This 3D image can be viewed from different perspectives, as if the object is actually there. When viewed from the front, the front of the object can be seen, and when viewed from the side, the side of the object can be seen. Most interestingly, when the hologram is cut apart, the image can still be viewed from perspectives that are recorded onto each piece. Because of this, and the fact that holograms can be viewed in different angles, holograms make a good candidate for data storage, providing means of storing mass amounts of data in different angles and maintaining data integrity even if the hologram is damaged. There is also another application for holograms. People at MIT have been hard at work, since 1989, in creating a "Holovideo" set that could replace your TV some day, just like Star Wars! Take a look at The Holovideo Page.
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