Interference Can Stop Things
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 CONSTRUCTIVE 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.