Toss a pebble in a pond -see the ripples? Now drop two
pebbles close together. Look at what happens when the two sets
of waves combine -you get a new wave! When a crest and a trough
meet, they cancel out and the water goes flat. When two crests
meet, they produce one, bigger crest. When two troughs collide,
they make a single, deeper trough. Believe it or not, you've
just found a key to understanding how a hologram works. But what
do waves in a pond have to do with those amazing three-
dimensional pictures? How do waves make a hologram look like the
real thing?
It all starts with light. Without it, you can't see. And
much like the ripples in a pond, light travels in waves. When
you look at, say, an apple, what you really see are the waves of
light reflected from it. Your two eyes each see a slightly
different view of the apple. These different views tell you
about the apple's depth -its form and where it sits in relation
to other objects. Your brain processes this information so that
you see the apple, and the rest of the world, in 3-D. You can
look around objects, too -if the apple is blocking the view of
an orange behind it, you can just move your head to one side.
The apple seems to "move" out of the way so you can see the
orange or even the back of the apple. If that seems a bit
obvious, just try looking behind something in a regular
photograph! You can't, because the photograph can't reproduce
the infinitely complicated waves of light reflected by objects;
the lens of a camera can only focus those waves into a flat, 2-D
image. But a hologram can capture a 3-D image so lifelike that
you can look around the image of the apple to an orange in the
background -and it's all thanks to the special kind of light
waves produced by a laser.
"Normal" white light from the sun or a lightbulb is a
combination of every colour of light in the spectrum -a mush of
different waves that's useless for holograms. But a laser shines
light in a thin, intense beam that's just one colour. That means
laser light waves are uniform and in step. When two laser beams
intersect, like two sets of ripples meeting in a pond, they
produce a single new wave pattern: the hologram. Here's how it
happens: Light coming from a laser is split into two beams,
called the object beam and the reference beam. Spread by lenses
and bounced off a mirror, the object beam hits the apple. Light
waves reflect from the apple towards a photographic film. The
reference beam heads straight to the film without hitting the
apple. The two sets of waves meet and create a new wave pattern
that hits the film and exposes it. On the film all you can see
is a mass of dark and light swirls -it doesn't look like an
apple at all! But shine the laser reference beam through the
film once more and the pattern of swirls bends the light to re-
create the original reflection waves from the apple -exactly.
Not all holograms work this way -some use plastics instead
of photographic film, others are visible in normal light. But
all holograms are created with lasers -and new waves.
All Thought Up and No Place to Go
Holograms were invented in 1947 by Hungarian scientist
Dennis Gabor, but they were ignored for years. Why? Like many
great ideas, Gabor's theory about light waves was ahead of its
time. The lasers needed to produce clean waves -and thus clean
3-D images -weren't invented until 1960. Gabor coined the name
for his photographic technique from holos and gramma, Greek for
"the whole message. " But for more than a decade, Gabor had only
half the words. Gabor's contribution to science was recognized
at last in 1971 with a Nobel Prize. He's got a chance for a last
laugh, too. A perfect holographic portrait of the late scientist
looking up from his desk with a smile could go on fooling
viewers into saying hello forever. Actor Laurence Olivier has
also achieved that kind of immortality -a hologram of the 80
year-old can be seen these days on the stage in London, in a
musical called Time.
New Waves
When it comes to looking at the future uses of holography,
pictures are anything but the whole picture. Here are just a
couple of the more unusual possibilities. Consider this: you're
in a windowless room in the middle of an office tower, but
you're reading by the light of the noonday sun! How can this be?
A new invention that incorporates holograms into widow glazings
makes it possible. Holograms can bend light to create complex 3-
D images, but they can also simply redirect light rays. The
window glaze holograms could focus sunlight coming through a
window into a narrow beam, funnel it into an air duct with
reflective walls above the ceiling and send it down the hall to
your windowless cubbyhole. That could cut lighting costs and
conserve energy. The holograms could even guide sunlight into
the gloomy gaps between city skyscrapers and since they can bend
light of different colors in different directions, they could be
used to filter out the hot infrared light rays that stream
through your car windows to bake you on summer days.
Or, how about holding an entire library in the palm of
your hand? Holography makes it theoretically possible. Words or
pictures could be translated into a code of alternating light
and dark spots and stored in an unbelievably tiny space. That's
because light waves are very, very skinny. You could lay about
1000 lightwaves side by side across the width of the period at
the end of this sentence. One calculation holds that by using
holograms, the U. S. Library of Congress could be stored in the
space of a sugar cube. For now, holographic data storage remains
little more than a fascinating idea because the materials needed
to do the job haven't been invented yet. But it's clear that
holograms, which author Isaac Asimov called "the greatest
advance in imaging since the eye" will continue to make waves in
the world of science.
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