The history of the hologram is not long. The first hologram was created by Dennis Gabor in 1947. He won a Nobel prize for it, but not until 1971. Since then, holography has evolved from an exotic science museum exhibit to everyday use in personal identification. If you have a credit card, you probably have a hologram.
The hologram is similar to a photograph only in that it provides an image of an object. How it provides that image is starkly different. A photograph simply provides a copy of the real image created from an object by the camera lenses. The light rays emanating from each unique point on the object are focused to one point on the film. A hologram, however, actually stores information about every unique point on the object at every point on the film. How is this done?
(Your text also provides a description of holography in Section 38.9. Check it out!)
Interference Patterns
What is stored on a holographic film exposure is not an image, but the interference pattern between light from the object and light from what is called the reference beam. As you know, a persistent interference pattern results from the addition of the waves from coherent sources. That is, the relative phase difference between the sources must be constant in time.
In order to achieve this, a laser beam is split into two beams, the object beam and the reference beam. The object beam is directed onto the object, where it reflects from the object and onto the film. The reference beam falls directly onto the film. If all distances are held fixed, there will be a persistent interference pattern recorded on the film. The spacing between the interference fringes is of the order of the size of the wavelength of the laser. So, a high resolution film is required.
In itself, this pattern is not an image of the object. Held up to ordinary light, the developed film will look like a complicated mishmash of overlapping circular patterns. But when a coherent reference beam is directed through the developed film, the interference pattern will diffract the beam, reproducing the original object beam for you to see. Every region on the film contains the interference pattern resulting from the original reference beam and the light reflected from every point on the object. If you cut a small section from the film, you will be able to see all of the object that was "in sight" of that section of the film. As you look through the film, you will see the original object beam light exactly as it was during the original exposure. The hologram created as described above is called a transmission hologram. The image is reproduced by transmitting coherent light through the film. But there are other types, such as the reflection hologram.
Making a Hologram
Making a hologram is a bit tricky, but it can be done for about $200. You can reduce that cost considerably if you use some of the equipment in the physics laboratory. The major elements required are :
Remember that your are creating an interference pattern. If any of the elements vibrate with amplitudes significantly large compared to the wavelength of the light, the interference pattern will be obliterated. Although that may seem an insurmountable obstacle, it's amazing what can be achieved with a steel plate or heavy plywood base resting on a large, soflty-inflated inner tube. Holgraphic Dimensions Inc has a very extensive site that provides background information on holography, as well as a how-to section. Mostly, what you need to make a hologram is a lot of patience. But that's cheap.
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