They finally became a reality in 1963 at the University of Michigan (United States), made possible by the invention of the laser (light concentrated into a coherent beam in which waves and photons propagate in phase). It was through electron optics that the engineer discovered the concept of holography, which he called wavefront reconstruction ("reconstruction by wavefronts"); the term now in common use was coined from the Greek adjective holos ("whole") and the verb graphein ("to write").
A hologram of an object consists of a transparent photographic plate on which the interference between two coherent laser beams originating from the same initial beam has been recorded. One of these beams is left unaltered, while the other is reflected by the object to be photographed. When the plate bearing the interference pattern is illuminated at a certain angle with a suitable beam of light, it recreates a three-dimensional image of the photographed object that seems to "float in mid-air."
