MadSci Network: Physics

Re: Cutting holographic film produces the same image?

Date: Thu Apr 10 18:28:49 2008
Posted By: Michael Richmond, Faculty, Physics, Rochester Institute of Technology
Area of science: Physics
ID: 1207744833.Ph

I will give you an example of very, very simplified "picture" which has been recorded onto holographic film. I hope that it might give you the general idea for how one can reconstruct the original image from a small piece of film.

I will assume that you know something about the diffraction of light and interference patterns; if you've taken an introductory physics course that covered waves, you should. If this is not familiar to you, you might read some of these references and play with the applets they provide:

So, let's create a very simple "object" and take a holographic picture of it. Our "object" will be a piece of cardboard with two narrow slits cut into it, a very small distance apart. To take a holographic picture, we shine a laser at the "object" -- in this case, it goes through the slits -- and the light falls onto photographic film on the other side.

When we develop the film, what will we see? We'll see a series of bright and dark bands. These bands are caused by the interference of light which went through the two slits. In a typical experiment, we might end up with 20 or 30 bright bands, all in a row, on a dark background.

Now, in order to reverse the procedure and create a reconstructed picture of the "object", we take the same laser and shine it through the film; that is, we shine the laser through the series of bands. The laser light is blocked by the dark background and the dark spaces between the bands, but it passes through the regions on the film which were the bright bands. If we allow the laser light to go through this film and fall on a distant wall, we will see (drum roll please) a pair of bright, narrow rectangles of laser light. Those rectangles are the reconstructed image of our original "object" -- which was, after all, just a pair of slits cut into cardboard.

The reason this happens is because when light passes through the series of many bright-and-dark bands, it interferes with itself in just the right way to create a pair of bright spots; just as light passing through two slits will interfere with itself to create a long series of bright-and-dark bands.

Okay, here comes the part you mentioned. If I shine light through the entire piece of film, which has 20 or 30 bright-and-dark bands, I will see a relatively sharp pair of spots on the distant wall. The larger the piece of film I have, and the larger the number of bands, the sharper the reconstructed image. If I now cover up some of the film -- or cut off a piece -- then I reduce the number of bands through which the laser light will travel. That will cause fewer laser light waves to interfere with each other, and so when the waves reach the distant wall, the pattern (two bright spots) will be less distinct. As long as there are _some_ bands, the waves will still interfere -- but not as completely as when there are many many bands.

That's how I understand the idea that a holographic image -- which is, in fact, the result of many waves of light bouncing off (or through) and object and then interfering with each other on a piece of film -- can be cut into pieces, yet still reveal a fuzzy version of the original image. I hope it helps you, too.

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