Toss a pebble in a pond -see the ripples? Now drop two
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
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
year-old can be seen these days on the stage in London, in a
called Time.

New Waves

When it comes to looking at the future uses of holography,
are anything but the whole picture. Here are just a
couple of