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.