So what are holograms? The best way to think of a hologram is to envision them as impressions on light waves.  Light is a wave. All waves behave more or less the same. For one thing they tend to echo. They reflect off of many surfaces. Sort of like sound waves echoing to make SONAR or microwaves in RADAR. The wave is sent out; it hits an object; it bounces back. Pretty simple idea. But  what you don’t think about is that when a wave bounces up against an object it takes its shape. Like pressing a piece of clay up against a key. The key leaves a three dimensional impression in the clay. Well if you imagine the clay as a light wave, basically the idea of holography is throwing the clay up against the key, having the key make an impression on the front of the clay, letting it bounce off, and finally storing the shape of the clay permanently.

 Now with SONAR or RADAR we are dealing with waves that are not visible.  You can’t see sound waves or microwaves. However with light waves we are working in the visible spectrum and consequently things that are visible are things that tend to record on photographic film. So in effect, a hologram is a photograph of the impression left on the surface of a light wave after it has bounced off an object.

Film Emulsion

 Lets look at a hologram recorded on silver halide film. What is film?  First there is a base material of clear plastic or glass. Then there is a very important layer that contains the photoreactive chemistry. They call it the emulsion. Its a very special composition. There’s always room for it. Its jello. Plain old gelatin, withoutany flavor or color of course. Inside the gelatin there are two chemicals joined in a molecule. The are suspended like fruit in jello.  In an emulsion, each chemical retains its own identity. Just like each piece of fruit floating in the jello does. First, there’s silver. As we all know silver has a very unique property. It tarnishes when it combines with oxygen. And it turns black. Next, there’s a halide like iodine, the stuff you use to kill the germs on a cut, and/or bromine, another halide that is used to bleach flour. Halides are very reactive chemicals.  So reactive that when they mix with the silver to make silver halide, it results in a silver that will tarnish very quickly.

 When a light wave goes into this layer of jello its energy is transferred to the silver halide molecule. Remember how a light wave looks like a corkscrew? Well, try to imagine this corkscrew winding up its energy into the silver halide molecule just like a wind-up toy. You give it a good twist and the energy goes into making the toy run. In the case of the silver halide molecule you give it a good wind up of light energy. Its sort of like setting a bear trap. First, you are putting your energy into pulling the trap open. Now its set to snap shut. The same thing is happening in the silver halide molecule. Light gives it energy to be ready to snap onto another atom. When you put it in a bath of photographic developer, it grabs hold of the oxygen in the bath, and tarnishes. That’s why black and white negatives are black. And so are holograms before they are bleached. So, you can think of photography or by extension holography as the art of selectively tarnishing silver in jello where light has energized it.

 Now in the case of a hologram, the patterns of light wave impressions are what is being photographed in the layer of emulsion. Generally film emulsion in holography runs about 10 microns thick. A micron being a millionth of a meter (a meter is about a yard) in size. That’s pretty small, but a photon measures about a half of a single micron in size.  That’s smaller than an ants asshole. So the emulsion seems pretty large to a photon. That’s how we are able to photograph this microscopic wave impression in film and make holograms. Holograms are photographs of the three dimensional impressions stored on light waves. Sort of like fossils.  When you pour plaster into a fossil, let it harden, and then remove it, you have a three dimensional sculpture of the impression that was left in the stone. Similarly, when you pour jello into a jello mold, let it set, and then remove it, you have a three dimensional sculpture of the shape of the jello mold. And when you put light into a hologram, you get a three dimensional sculpture in light of the object that left its impressions on a photon and was captured within the thickness of a photographic emulsion.