Holography is a technique for recording and reproducing an image of an object without the use of lenses.Unlike the two-dimensional images recorded by an ordinary photograph or a television system，a holographic image is truly three-dimensional.Such an image can be viewed from different directions to reveal different views，and from various distances to reveal changing perspective.

　　The basic procedure for making a hologram is very simple in principle.A possible arrangement is shown in fig.8-12a.We illuminate the object to be holographied with a monochromatic light，and we place a photographic film so that it is struck by scattered light from the object and also by direct light from the source.In practice，the source must be a laser，for reasons to be discussed later.Interference between the direct and scattered light leads to the formation and recording of a complex interference pattern on the film.

　　To form the images，we simply project laser light through the developed film，as shown in fig.8-12b.Two images are formed，a virtual image on the side of the film near the source，and a real image on the opposite side.