Holography

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Holography

Method of obtaining three-dimensional photographic images. These images are obtained without a lens, so the method is also called lensless photography. The records are called holograms derived from Greek holos, meaning “whole” and gram, meaning “message”. The theoretical principles of holography were developed by the British physicist Dennis Gabor in 1947. The first actual production of holograms took place in the early 1960s, when the laser became available. By the late 1980s, the production of true-color holograms was possible, as well as holograms ranging from the microwave to the X-ray region of the spectrum. Ultrasonic holograms were also being made, using sound waves.

Production,

A hologram differs essentially from an ordinary photograph in that it records not only the intensity distribution of reflected light but also the phase distribution. That is, the film distinguishes between waves that reach the light-sensitive surface while they are at maximum wave amplitude, and those that reach the surface at minimum wave amplitude. This ability to discriminate between waves with different phases is obtained by having a so-called reference beam interfere with the reflected waves.

Thus, in one method of obtaining a hologram, the object is illuminated by a beam of coherent light—a beam in which all the waves are traveling in phase with one another. Such a beam is produced by a laser. Essentially, the shape of the object determines the form of the wave fronts—that is, the phase at which the reflected light arrives on each point of the photographic plate. Simultaneously, a portion of the same laser beam is reflected by a mirror or prism and directed toward the photographic plate; this beam is called the reference...

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...ath parallel to the earth's direction of motion would take longer to pass through a given distance than light traveling the same distance in a path perpendicular to the earth's motion. The interferometer was arranged so that a beam of light was divided along two paths at right angles to each other; the rays were then reflected and recombined, producing interference fringes where the two beams met. If the hypothesis of the ether were correct, as the apparatus was rotated the two beams of light would interchange their roles (the one that traveled more rapidly in the first position would travel more slowly in the second position), and a shift of interference fringes would occur. Michelson and Morley failed to find such a shift, and later experiments confirmed this. Today the propagation of electromagnetic waves through empty space has replaced the concept of the ether.

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