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332 Lasers
be achieved by a second excitation in which they become ionized and can be
collected by an electric field.
A disadvantage of the process is that, once perfected, it will enable do-it-
yourself enthusiasts (with possibly a sprinkling of terrorists among them) to
make their own atomic bombs.
12.13.18 Holography
I would like to mention holography, a method of image reconstruction inven-
ted by Dennis Gabor in 1948 (Nobel Prize, 1973). It is difficult to estimate
at this stage how important it will eventually turn out to be. It may remain
for ever a scientific curiosity with some limited applications in the testing of
materials. On the other hand it might really take off and might have as much
influence on life in the twenty-first century, as the nineteenth century invention
of photography has upon our lives. The technique is by no means limited to
the optical region; it could in principle be used at any frequency in the elec-
tromagnetic spectrum, and indeed, holography can be produced by all kinds
of waves including acoustic and electron waves. Nevertheless, holography
and lasers became strongly related to each other, mainly because holographic
image reconstruction can most easily be done with lasers at optical frequencies.
The basic set-up is shown schematically in Fig. 12.27. The laser beam is
split into two, and the object is illuminated by one of the beams. The so-called
‘hologram’ is obtained by letting the light scattered from the object interfere
with the other beam. The pattern that appears depends both on the phase and
Laser
Mirror
Semi-transparent Lens
mirror
Hologram
Object
Fig. 12.27
Lens
Schematic representation of taking a
hologram. Mirror