Holograms

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