An object is illuminated with a beam of coherent light [object beam]. Then every point on the surface of the object acts as a source of secondary waves. These secondary waves spread in all directions. Some of these waves are allowed to fall on a recording plate [holographic plate]. Simultaneously, another beam of same coherent light [reference beam] is allowed to fall on this holographic plate. In the holographic plate, both the beams combine and interference pattern will be formed. This interference pattern is recorded on the holographic plate. The three-dimensional image of the object can be seen by exposing the recorded holographic plate [hologram] to coherent light. This is the principle of holography.