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 light bulb 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 with outhitting 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 holds and gram ma, Greek for 'the whole message. ' But for more thana 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 " rereading by the light of the noonday sun! How canthus be? A new invention that incorporates holograms into widow glazing's makes it possible. Holograms can bend light to create complex 3- Damages, 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 light waves 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.