HologramsToss a pebble in a pond -see the ripples?Now drop two pebbles closetogether. Look at what happens when the two sets of waves combine -you get anew wave! When a crest and a trough meet, they cancel out and the water goesflat. When two crests meet, they produce one, bigger crest.
When two troughscollide, they make a single, deeper trough. Believe it or not,you’ve justfound a key to understanding how a hologram works. But what do waves in apond have to do with those amazing three-dimensional pictures? How dowaves make a hologram look like the real thing?It all starts with light. Without it, you can’t see. And much like theripples 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 eyeseach see a slightly different view of the apple.These different viewstell you about the apple’s depth -its form and where it sits in relation toother 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 theapple is blocking the view of an orange behind it, you can just move yourhead to one side. The apple seems to “move” out of the way so you cansee the orange or even the back of the apple.If that seems a bitobvious,just try looking behind something in aregular photograph!You can’t, because the photograph can’t reproduce the infinitely complicatedwaves of light reflected by objects; the lens of a camera can only focus thosewaves into a flat, 2-D image.
But a hologram can capture a 3-D image solifelike that you can look around the image of the apple to an orange in thebackground -and it’s all thanks to the special kind of light waves producedby a laser.”Normal” white light from the sun or a lightbulb is a combinationof every colour of light in the spectrum -a mush of different waves that’suseless for holograms. But a laser shines light in a thin, intense beam that’sjust one colour. That means laser light waves are uniform and in step. Whentwo 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 beamand the reference beam. Spread by lenses and bounced off a mirror, the objectbeam hits the apple. Light waves reflect from the apple towards a photographicfilm.The reference beam heads straight to the film without hitting theapple. The two sets of waves meet and create a new wave pattern that hits thefilm and exposes it. On the film all you can see is a mass of dark and lightswirls -it doesn’t look like an apple at all! But shine the laserreference beam through the film once more and the pattern of swirls bends thelight to re-create the original reflection waves from the apple -exactly.Not all holograms work this way -some use plastics instead ofphotographic film, others are visible in normal light.But all holograms arecreated with lasers -and new waves.
All Thought Up and No Place to GoHolograms were invented in 1947 by Hungarian scientist Dennis Gabor,but they were ignored for years. Why? Like many great ideas, Gabor’s theoryabout light waves was ahead of its time. The lasers needed to produce cleanwaves -and thus clean 3-D images -weren’t invented until 1960. Gabor coinedthe name for his photographic technique from holos and gramma, Greek for “thewhole 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 NobelPrize. He’s got a chance for a last laugh, too.
A perfect holographic portraitof the late scientist looking up from his desk with a smile could go onfooling viewers into saying hello forever. Actor Laurence Olivier has alsoachieved that kind of immortality -a hologram of the 80 year-old can beseen these days on the stage in London,in a musical called Time.New WavesWhen it comes to looking at the future uses of holography, pictures areanything but the whole picture.Here are just a couple of the more unusualpossibilities.
Consider this: you’re in a windowless room in the middle ofan office tower,but you’re reading by the light of the noonday sun! Howcan this be? A new invention that incorporates holograms into widow glazingsmakes it possible. Holograms can bend light to create complex 3-D images, butthey can also simply redirect light rays.The window glaze holograms couldfocus sunlight coming through a window into a narrow beam, funnel it intoan air duct with reflective walls above the ceiling and send it down thehall to your windowless cubbyhole. That could cut lighting costs andconserve energy. The holograms could even guide sunlight into the gloomygaps between city skyscrapers and since they can bend light of different colorsin different directions, they could be used to filter out the hot infraredlight 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 betranslated into a code of alternating light and dark spots and stored in anunbelievably tiny space.That’s because light waves are very, very skinny.
You could lay about 1000 lightwaves side by side across the width of theperiod at the end of this sentence. One calculation holds that by usingholograms, the U. S. Library of Congress could be stored in the space of asugar cube.
For now, holographic data storage remains little more than afascinating idea because the materials needed to do the job haven’t beeninvented yet.But it’s clear that holograms,which author Isaac Asimovcalled “the greatest advance in imaging since the eye” will continue to makewaves in the world of science.Science