## Making a hologram

Light from a laser passes through a beam splitter. One half (the reference beam) is reflected through 90°, passes through a diverging lens and then continues on to illuminate holographic film at the right in Figure 9.4. The other half (the object beam) goes straight through the beam splitter and is then reflected back to another diverging lens to illuminate the object (e.g. chess pieces). The beam is reflected from the object to merge with the reference beam at the holographic plate.

In Figure 9.5 the photographic plate has been developed and the hologram is held approximately in the same place as before. The chess board and chess pieces are gone, and so is the beam splitter, leaving only the reference beam. Looking through the hologram we still see the chess pieces as a three-dimensional image. The right hand picture is a photograph of what we see in the hologram — the 'ghosts of the chess game'.

Figure 9.4 Making a hologram.
Figure 9.5 The finished product. Courtesy of James Ellis, School of Physics, UCD, Dublin.

The developed holographic film bears no resemblance to a 'normal' photograph. It consists of a pattern of irregular black and white fringes. These are interference fringes, which are very different from, for example, those in Young's slits experiment, because the wave fronts of the light reflected from the object are irregular.

The brightness at any place on the film is not only determined by the intensity of the reflected light but also by the path differences between the object and reference beams. The distances travelled by light from different parts of the object depend on its shape, so the interference pattern is an encoded map of the surface of the object. The image is virtual.

9.2.4 Why does a holographic image look so real?

As we look at the image from different angles we see different sides of the object — a complete three-dimensional picture! This is of course completely different from a photograph, where you see only part of the image on any one piece of the film. (In Figure 9.6, the angular separation between view 1 and view 2 is about 45°; in practice the separation is considerably smaller.)

The holograms we have described are transmission holograms, made by passing the object beam and the reflected beam through the film in the same direction. Reflection holograms are made by passing the object beam and reference beams through the film in opposite directions. They are viewed by reflecting light from the surface of the hologram.

The first 'white light' hologram, a reflection hologram, was produced in 1947, and the first white light transmission hologram was produced in 1968. These developments brought holography to the attention of the general public.

### 9.2.5 Applications of holography

Holographic techniques are very widely used. Well-known applications are the lasers scanners used at supermarket checkout counters and the embossed images used on credit cards. Medical applications include holographic imaging of the interior of inside

'ghost'image light from Ok " horse's nose

view 1 view 2

Figure 9.6 Viewing the image from different sides.

view 1 view 2

### Figure 9.6 Viewing the image from different sides.

live organs using optical fibres and three-dimensional CT (computed tomography) scanning. Recent years have seen the development of materials for holographic storage of computerised data.

### The Lindow man

Holograms may be used to store information about very fragile objects, such as museum artifacts. In 1984, the remains of an Iron Age man were found in Lindow Moss (a peat bog in Cheshire, England). Holographic images were used to construct a model of his original features.

### Holograms from other waves

Holographic imaging is not restricted to light waves. Any electromagnetic radiation and even sound waves can also be used to make holograms. Ultraviolet and X-ray holograms have higher resolution than visual holograms.

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