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Holograms are photographs of three dimensional impressions on the
surface of light waves. Therefore, in order to make a hologram you need
to photograph light waves. This presents something of a dilema.
As we all know, it can be problematic to take a photograph of a quickly
moving object. If you've ever had a picture come back blurred from the
film lab, you know all too well. When a person moves too quickly in a
photograph, their image blurs. And they are only moving at about 20
miles an hour. Try to imagine the problems associated with trying to
photograph a photon. To start, a light wave moves at the speed of
light. Thats about 186,000 miles per second. Thats more than half way
to the moon in a second. Considerably faster than someone's hand
waving. In fact, its so fast that the very idea of even capturing it on
film would appear impossible. What we need is a way to stop the photon
so it can be photographed. And this technique is called INTERFERENCE.
Imagine yourself standing on a small bridge over a pond of still water.
Lets further imagine that you were to drop a pebble into the pond. As
it hits the water it creates a circular wave. This wave radiates
outwards in an ever growing circular path. We've all done this.
Now, if you drop two pebbles in the water, you would create two circular
waves, each of which would grow in size and eventually cross the path of
the other wave and then continue on its individual expanding path.
Where the two circular waves cross each other, you might say that they
interfere with each other. And the pattern that they make is called an
interference pattern. Not too difficult to envision. This is what
interference is. Two waves interfering with each other as they cross
paths. No permanent impact is left on either wave once it leaves the
area of overlap. Each wave looks exactly the same as it did before it
crossed the other waves path. Well, maybe its grown a little bit
bigger, but that's about it. So, what's the big deal about interference
in that case?
Here it is. As waves cross paths and interfere, the pattern they make
is called a standing wave. It is called a standing wave because it
stands still. And since it stands still, it can be photographed.
This solves the problem of how we can photograph something moving at the
speed of light. But it doesn't answer the bigger question. Why does it
stand still?
To understand that, lets envision a photon. Remember? It looks like a
corkscrew. And if we view it from the side it looks like a sine wave.
Now, try to imagine a river whose streambed lies on a wavy rock
formation that looks like a sine wave. This river would be full of
rapids. In fact, it would be great for white water rafting. Although
the water in the river is flowing furiously downstream, the pattern of
water above the rapids is stationary. You might think of it as a
standing wave. The wave energy is flowing through this standing wave
without altering it and vice versa. It is just a momentary pattern that
the water takes as it passes over a bump.
When two light waves pass through each other each wave acts like a bump
to the other. Their respective corkscrew shapes interact. And the
result is like rapids of light. The standing wave patterns are
stationary even though the light waves energy continues to move.
When waves meet they perform addition and subtraction. When two waves
of equal size meet at their high points (called crests), they add
together to make a wave twice as high at that point. Conversely, where
two waves of equal size meet at their low points (call troughs) they add
together to become twice as low. And when one wave at its high point
meets another wave at its low point they subtract and cancel out. But
it isn't really cancelled out in the sense of being destroyed. Its more
a case of there being no light at that spot. If you follow the wave
down its path just a drop further it will be meeting the other wave at a
different relationship and once again be visible. Its a situation of
infinite possibilities. Just like the patterns possible as the waves of
two pebbles meet in a pond. At any point you may notice that the
standing wave pattern has produced a place where the waves have added
together to get higher or subtracted to become lower or even just gone
flat. There's a few terms that are used to describe the possible
encounters. If the waves add and get higher its called CONSRTRUCTIVE
interference. If the waves subtract or cancel altogether its called
DESTRUCTIVE interference.
I like to think of the interference pattern as a fingerprint of the
encounter of two individual waves. Each object you make a hologram of
creates its own interference pattern that identifies it.
In holography, there are two basic waves that come together to create
the interference pattern. First and foremost is the wave that bounces
off the object we are making a hologram of. Since it bounces off the
object, thereby taking its shape, it is called the OBJECT wave. You
can't have interference without something to interfere with. So a
second wave of light that has not bounced off an object is used to
perform this function. It is called the REFERENCE wave.
When an object wave meets a reference wave creating a standing wave
pattern of interference, it is photographed and called a hologram.
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