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Physical principles

Glossary FAQ

There are two physical phenomena as the principles of the holography: interference and diffraction of light waves.
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 dilemma.Interference pattern 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. 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 300.000 kilometers 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 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 theLaser device problem of how we can photograph something moving at the speed of light. So, to photograph interference pattern we should use special light source. It is laser, which was first made to operate in 1960.

Laser light differs drastically from all other light sources, man-made or natural, in one basic way which leads to several startling characteristics. Laser light can be coherent light. Ideally, this means that the light being emitted by the laser is of the same wavelength, and is in phase. These might be new terms for some of you, so let us form an analogy that might clarify the term coherence.

But we don't answer the bigger question. Why does light wave stand still? To understand that, let's envision a photon. 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.-->