How A Hologram Works
Holograms are made by bouncing laser light off an object. When the wave of light hits the object, its just like taking a piece of
modeling clay and pressing it up against that object. It takes the shape of the object. Once it has taken the shape it bounces off and onto a piece of photographic film. Just like the film that is used in a disposable camera, but
much more able to capture even the tiniest details. Film captures images by recording where light hits it. So, what is film?
Film is made of a clear piece of plastic covered with gelatin (just like clear jell-o with no flavor).
Inside the gelatin are grains of silver iodide (sometimes called silver halide). These grains are light sensitive. When light waves hit these grains,
it gives them energy. Just like the light energy was pulling it open in the
same way someone might pull open a bear trap. The exposed film is put in a bath of a liquid chemical called developer. The silver halide grains that have been exposed to light, grab on to oxygen in
the developer chemicals. When a silver based chemical mixes with oxygen it turns black. Your Mom can tell you how her silver jewelry tarnishes. Tarnish is the word people use to describe the way
silver turns black when oxygen mixes with it. Holographic film can capture details thousands of times smaller than regular film. That's because the grains
of silver halide in regular photographic film are much bigger than the grains in holographic film. Its like the difference in size of the grains of table sugar compared to the powder fine grains
of confectioner's sugar that coats a powdered sugar donut. The name for this ability to record fine details in film is called resolution. The smaller the size of the grains
the better the resolution. We need this ultra fine resolution in holography
because our pictures are of microscopic light wave patterns.
Its a lot like having a real lot of pixels in a digital camera. The more pixels, the more detail.
But there's another step in making holograms. And its a very important one. We need to make the light waves stand still in order to take their picture. The method we use is
According to one of the smartest scientist who ever lived, Albert Einstein, light is the fastest moving thing in the universe. Now, this poses a problem. How do you take a picture of something moving
that fast? I'm sure you have taken pictures of someone moving that have come out blurry. And they were moving much slower than light speed. So, how is it done?
Light travels in waves. Just like waves in the ocean. All waves behave the same. When waves meet in the ocean, they cross paths. You might say that as the waves cross, they interfere with
one another. Imagine yourself standing on a small bridge over a pond with Albert Einstein. He might have suggested that both of you drop a small stone into the water at the same time. As the stones hit the water they splash and make
circular waves that spread out. Soon the two circular waves meet and cross over each other. In time the waves pass each other and continue moving on their merry way. But what we're interested in is the area in which they meet and
interfere. Something special happens there. The pattern of interfering waves stand still! So lets try to understand why this happens.
Think of the way a wave looks in the water. Its got a high point and then a low point. You might say its got a bump in it. When one wave travels over another wave it has to leap frog over the bumps it sees in the other wave. The bumps are like
what happens when a river has to leap frog over big boulders in its path. You see big white water sometimes called rapids. From the side of the river these rapids look like bumps in the swiftly running water. The water is moving through the bump,
but the bump stays in the same place. It stands still. Scientists call a wave that stands still a standing wave. So, whenever waves interfere, each wave sees the other in the same way the water in a river sees the boulders in its
bed. Its a series of bumps it leap frogs over and creates standing waves. And since the waves stand still, we can take a photograph of them. We call this photograph of standing waves a hologram.