(D). Application Three-dimensional imaging by Holography:
Holography is lens less three-dimensional photography. A conventional photograph is only a flat record of a real image projected onto a photographic film. Information about the three dimensional character of the object is almost entirely lost during the photographic recording process. A hologram on the other hand is a special 'photograph' of an object that retains information about the phase of waves coming from the object. Holography invented by Dennis Gabor in 1947. had not led to applications until lasers became available. Today, it is used in a multitude of ways including three-dimensional representation of objects, fingerprint identification and laser beam diffraction scanning.
A laser can be used to make a special type of photographic image called a hologram. Fig.19 shows the basic principle. The hologram is formed on the photographic film. It doesn't look like a 'real' picture. It is an interference pattern, produced when waves direct from the laser meet waves reflected from the chess piece. If, later, laser light is shone back through the hologram, a three-dimensional image of the chess piece can be seen.
Not all holograms need laser light to give an image. Some work using reflected daylight. Credit cards often have reflection holograms printed on them as shown in Fig.20. They make the cards very difficult for forgers to copy.
