HologramsToss a pebble in a pond -see the ripples?Now drop twopebbles close together. Look at what happens when the two setsof waves combine -you get a new wave! When a crest and a troughmeet, they cancel out and the water goes flat. When two crestsmeet, they produce one, bigger crest. When two troughs collide,they make a single, deeper trough. Believe it or not,you’vejust found a key to understanding how a hologram works. But whatdo waves in a pond have to do with those amazing three-dimensional pictures? How do waves make a hologram look like thereal thing?It all starts with light. Without it, you can’t see. Andmuch like the ripples in a pond, light travels in waves.Whenyou look at, say, an apple, what you really see are the waves oflight reflected from it. Your two eyes each see a slightlydifferent view of the apple.These different views tell youabout the apple’s depth -its form and where it sits in relationto other objects. Your brain processes this information so thatyou see the apple, and the rest of the world, in 3-D. You canlook around objects, too -if the apple is blocking the view ofan 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 theorange or even the back of the apple.If that seems a bitobvious,just try looking behind something in aregularphotograph! You can’t, because the photograph can’t reproducethe infinitely complicated waves of light reflected by objects;the lens of a camera can only focus those waves into a flat, 2-Dimage. But a hologram can capture a 3-D image so lifelike thatyou can look around the image of the apple to an orange in thebackground -and it’s all thanks to the special kind of lightwaves produced by a laser.”Normal” white light from the sun or a lightbulb is acombination of every colour of light in the spectrum -a mush ofdifferent waves that’s useless for holograms. But a laser shineslight in a thin, intense beam that’s just one colour. That meanslaser light waves are uniform and in step. When two laser beamsintersect, like two sets of ripples meeting in a pond,theyproduce a single new wave pattern: the hologram. Here’s how ithappens: Light coming from a laser is split into two beams,called the object beam and the reference beam. Spread by lensesand bounced off a mirror, the object beam hits the apple. Lightwaves reflect from the apple towards a photographic film.Thereference beam heads straight to the film without hitting theapple. The two sets of waves meet and create a new wave patternthat hits the film and exposes it. On the film all you can seeis a mass of dark and light swirls -it doesn’t look like anapple at all! But shine the laser reference beam through thefilm 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 insteadof photographic film, others are visible in normal light.Butall holograms are created with lasers -and new waves. All Thought Up and No Place to GoHolograms were invented in 1947 by Hungarian scientistDennis Gabor, but they were ignored for years. Why? Like manygreat ideas, Gabor’s theory about light waves was ahead of itstime. The lasers needed to produce clean waves -and thus clean3-D images -weren’t invented until 1960. Gabor coined the namefor his photographic technique from holos and gramma, Greek for”the whole message. ” But for more than a decade, Gabor had onlyhalf the words. Gabor’s contribution to science was recognizedat last in 1971 with a Nobel Prize. He’s got a chance for a lastlaugh, too. A perfect holographic portrait of the late scientistlooking up from his desk with a smile could go on foolingviewers into saying hello forever. Actor Laurence Olivier hasalso achieved that kind of immortality -a hologram of the 80year-old can be seen these days on the stage in London,in amusical called Time. New WavesWhen it comes to looking at the future uses of holography,pictures are anything but the whole picture.Here are just acouple of the more unusual possibilities. Consider this: you’rein a windowless room in the middle of an office tower,butyou’re reading by the light of the noonday sun! How can this be?A new invention that incorporates holograms into widow glazingsmakes it possible. Holograms can bend light to create complex 3-D images, but they can also simply redirect light rays.Thewindow glaze holograms could focus sunlight coming through awindow into a narrow beam, funnel it into an air duct withreflective walls above the ceiling and send it down the hall toyour windowless cubbyhole. That could cut lighting costs andconserve energy. The holograms could even guide sunlight intothe gloomy gaps between city skyscrapers and since they can bendlight of different colors in different directions, they could beused to filter out the hot infrared light rays that streamthrough your car windows to bake you on summer days.Or, how about holding an entire library in the palm ofyour hand? Holography makes it theoretically possible. Words orpictures could be translated into a code of alternating lightand dark spots and stored in an unbelievably tiny space.That’sbecause light waves are very, very skinny. You could lay about1000 lightwaves side by side across the width of the period atthe end of this sentence. One calculation holds that by usingholograms, the U. S. Library of Congress could be stored in thespace of a sugar cube. For now, holographic data storage remainslittle more than a fascinating idea because the materials neededto do the job haven’t been invented yet.But it’s clear thatholograms,which author Isaac Asimov called “the greatestadvance in imaging since the eye” will continue to make waves inthe world of science.