A hologram is made in the following manner and is schematically shown in the figure. A parallel, monochromatic, coherent light beam is split into two beams by the help of a partially reflecting mirror. One of them is made incident on the object of which the hologram is required and the reflected beam is made to interfere with the first beam. The interference pattern is recorded by a photographic recording process. The recording procedure can be different for a transmission hologram and a reflection hologram. Here what is recorded is the interference pattern rather than the intensity distribution as in a photograph. This interference pattern, of course, refers to the colour (or wavelength) of the light used for making the hologram, but it would act as a diffraction grating for any other wavelength as well. The constructive interference condition for different wavelengths being different, there occurs dispersion of the diffracted light. Thus, when white light is incident on the hologram, the diffraction splits the different colours constituting the incident light.
