Holograms

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