The idea of holography was developed by Dennis Gabor in 1948 but it required the invention of lasers in the 1960's-with their coherence and intensity-for the actual production of a hologram. Figure 20.21 is a schematic of an arrangement used to make a hologram. Light from a laser is passed through a lens, called a beam spreader, to spread the beam so it will illuminate a larger area instead of remaining in its usual pencil-thin column. The light then passes through a partially silvered mirror. Some of the light passes through; this is called the reference beam. Some of the light is reflected and illuminates the object. A detailed and complicated wave is then reflected from the object. This wave produces a complex interference pattern where in meets with and interferes with the reference beam. This complex interference pattern is recorded on a photographic plate. If you look at a hologram, you may see dots and circles and swirls. These are due to extraneous dust or anomalies in the laser or the beam spreader; they are not part of the hologram itself. The details of the interference pattern which is the hologram itself is microscopic in size.

The interference pattern on the photographic plate is then developed. The laser and beam spreader are then used to illuminate the plate or hologram as in Figure 20.22. Light coming from this complicated microscopic pattern produces a wave coming from the plate which is identical to the wave that came from the original object. This means that as your eye intercepts these waves, we see an image just like the original object. As you move your head from side to side, you will see different perspectives just as if you were looking at the original. This is illustrated in Figure 20.23.

Variations of this procedure allow holograms to be viewed under ordinary, incoherent light. Color holograms are also possible. Holograms can also be produced that show movement as the perspective changes. Holograms can be used to look at the interference patterns of vibrating musical instruments. Such holograms are called interferograms.