Holograms

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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.

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