holography It is a two step process for recording and viewing of images with three dimensional perspective. An ordinary photograph records the information on the amplitudes of waves but information about the relative phases of the waves from different regions is lost. Gabor in 1971, devised a method, whereby both amplitude and phases of the wavefronts are preserved on a photographic plate known as hologram.

Consider interference of a plane reference wave with spherical wavefronts that are scattered from a point object( fig. h2a). The interference fringes are recorded on thin plate P parallel to the reference wavefront. The interference pattern looks similar to Newton's rings. (fig. h2b). When the plate is illuminated by a coherent plane wavefront of the reference wave. it acts as grating with variable spacing, like that of zone plate. When a parallel beam of light falls on it, two diffracted beams are produced, one which converges at I2, and the other appear to diverge from I1. On viewing from right we see the virtual image I1 where the point object was situated (fig. h3).

Any finite sized object is collection of point sources from which spherical waves are emitted or scattered. A hologram of such an object can reconstruct exact replica of the original waves from the object itself giving a virtual image with 3 dimensional perspective.

Some important uses of holographic technique are, information storage, many forms of non destructive testing, determination of particle sizes etc.