Holograms – what are they?
Recently I bought an item from mps called an i balance wrist band. This is an energy bracelet, similar to those worn by many people in sport, with several holograms and strong magnets beneath those holograms. Now we all think we know what holograms are – until some one asks us to explain it. So in this article I discuss what a hologram actually is.
Holography (the technique for creating holograms), is a method of using coherent light to record and display a three-dimensional image. A hologram is created by taking a beam of coherent light and shining part of it onto the object being recorded and half of it onto the recording medium. (Coherent light means light of a fixed wavelength in which all the waves are in step with one another. The best source of such light is a laser.)
The light that hits the object is scattered and some of it hits the medium (e.g. special film) being used to record the object. Because the light is coherent and comes from two sources, this created something called an interference pattern. What this means is that where a crest (top) of one wave from one of the sources of light hits the medium at the same time and place as the trough (bottom) of a light wave from the other source, they cancel each other out. On the other hand when two crests or two troughs hit the recording medium at the same time, they reinforce each other.
This produces a pattern on the recording medium that may in fact look nothing like the original object being recorded. It is this pattern that is called a hologram. When another beam of coherent light of the same wavelength is shone onto hologram, and the light is scattered back onto the retina of the eye of some one viewing it, the image of the original object is recreated and the viewer sees it as a three dimensional object.
It is also possible to shine a bright non-coherent light onto a hologram and see some semblance of the original image, by tilting it this way and that until the image becomes visible. The reason for this is that although the light is non-coherent as a whole, it is likely to contain some coherent light in it. Under these circumstances, one does not usually see the three dimensional image, but only a two dimensional version of it.
Indeed if you record a hologram on a glass place and then break the glass plate, each piece of the broken glass will contain the whole two-dimensional version of the image.
A couple of interesting side-notes to this. When holography was invented by Anglo-Hungarian physicist Denis Gabor in 1948, his original purpose was to improve the images in electron microscopes. It was only from 1960 onwards that it became possible to use holography with light to record everyday objects. But since then holography has found a use in entertainment, security (bank cards, passports, security passes) and alternative medicine. Magnetic bracelets of silicon with embedded hologram have become the new growth area in personal complementary medicine.