HOLOGRAPHY

When we see around us we can feel that the objects are in three dimensions that is, we have the ability to analyze the intensity distribution and the depth of the different points of an object. In other words we have the ability to analyze the amplitude variation and path difference between the two points. When a photograph of an object is taken all that we see is the amplitude variation but no path difference. This leads to a two dimensional view. In view of this, there is a necessity that a technique has to be developed, such that when photograph is taken it should have the information about amplitude variation and path difference between the two points. This is what is known as Holography. The information about the path difference between the two points is nothing but the information regarding the phase difference between the two rays coming from two different points, That is, to have a holography what that we need is the information regarding amplitude variation and phase variation. The only experiment which can give us both these information is an interference experiment and hence the techniques of holography has to essentially depend upon the interference phenomena.
Some times some sketches gives us a three dimensional feel and also with the help of specially prepared spectacle, one can get three dimensions feeling. This should not be understood as holography, they are just illusions in reality.
In principle holography can be had using multiple wave lengths of source but a single color will give a better information regarding the phase difference and hence use of monochromatic source of light is preferable. Further, whenever a difference is measured if the reference is uniform, the measurement becomes clearer. Hence, a laser as a source becomes very important in holography because not only it is monochromatic but also it has both the phase and temporal coherence.