Data Bases • Custom Term Papers • Free Term Papers • Free Research Papers • Free Essays • Free Book Reports • Plagiarism? Links • Top 100 Term Paper Sites • Top 25 Essay Sites • Top 50 Essay Sites • Search 97,000 Papers @ DirectEssays.com • Search 101,000 Papers @ ExampleEssays.com • Search 90,000 Papers @ MegaEssays.com Free Essays • Term Paper Sites • Chuck III's Free Essays • Free College Essays • TermPaperSites.com • My Term Papers • Get Free Essays • Essay World • Planet Papers	Search Lots of Essays Back to Subjects - Physics Holography 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. 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 beam. The wave fronts of this latter beam, not having been reflected from the object, remain plane-parallel and produce an interference pattern with the wave fronts of the light reflected by the object. If the object is a point, for example, the wave fronts of the reflected beam will be spherical; the interference pattern produced on the film will then consist of concentric circles, the space between circles decreasing with increasing radius. The interference pattern produced by a more complicated object will be much more complicated, so mere inspection of the resulting hologram will reveal only an intricate pattern of dark and light structures that bear no apparent relationship to the original object. When the hologram is viewed in coherent light, however, the recorded object becomes visible; and when the hologram is viewed from different angles, the object is also seen from different angles. The three-dimensional effect is obtained because the hologram reconstructs in space the wave fronts that originally were produced by the object. How this happens can be understood by again using the example of the hologram of the point. Coherent light arriving at the concentric circles on the hologram is diffracted on a diffraction grating. The diffraction angle of the beam increases with the distance from the center of the concentric rings, thus reconstructing the spherical wave fronts, and the viewer sees the point at the same relative place where the real point was when the hologram was made. The wave fronts of more complicated objects are reconstructed in the same way. The intensity distribution of the reflected light is recorded in the degree of blackening of the interference patterns on the film. To a certain extent, holography can be applied in optical microscopy, especially for the study of living organisms. The most successful application of holography, however, is in interferometry. If two holograms of the same object are recorded on the same plate, then upon reconstruction the two holographic images will interfere. If the object has undergone a deformation between the two recordings, phase differences in certain parts of the two images will result, creating an interference pattern that clearly shows the deformation. Because wave front differences of a fraction of a wavelength of light thus become visible, this method is extremely sensitive for deformation studies. Another important application is the storage of digital data, which can be recorded as bright and dark spots in holographic images. A hologram can contain a large number of “pages” that are recorded at different angles relative to the plate, thus allowing the storage of a very large amount of data on one hologram. By illuminating the hologram with a laser beam at different angles, the pages can be read out one by one. Instrument that utilizes the phenomenon of interference of light waves for the precise measurement of wavelengths of light itself, of small distances, and of certain optical phenomena. Because the instrument measures distances in terms of light waves, it permits the definition of the standard meter in terms of the wavelength of light. Many forms of the instrument are used, but in each case two or more beams of light travel separate optical paths, determined by a system of mirrors and plates, and are finally united to form interference fringes. In one form of interferometer for measuring the wavelength of monochromatic light, the apparatus is so arranged that a mirror in the path of one of the beams of light can be moved forward through a small distance, which can be accurately measured, thus varying the optical path of the beam. Moving the mirror through a distance equal to one-half of the wavelength of the light causes one complete cycle of changes in the pattern of interference fringes. The wavelength is calculated by measuring the number of cycles caused by moving the mirror through a measured distance. When the wavelength of the light used is known, small distances in the optical path can be measured by analyzing the interference patterns produced. This technique is used to measure the surface contours of telescope mirrors. The refractive indices of substances are also measured with the interferometer, the refractive index being calculated from the shift in interference fringes caused by the retardation of the beam. The principle of the interferometer is also used to measure the diameter of large stars, such as Betelgeuse. Because modern interferometers can measure very tiny angles, they are further used—again, on such nearby giants as Betelgeuse—to gain images of actual brightness variations on the surfaces of such stars. This technique is known as speckle interferometry. The interferometer principle has also been extended to other wavelengths, and it is now widely employed in radio astronomy. Historically, the best-known interferometer is the one devised about 1887 by the American physicist Albert Michelson for an experiment he conducted with the American chemist Edward Morley. The experiment was designed to measure the absolute motion of the earth through a hypothetical substance called the ether, erroneously presumed to exist as the carrier of light waves. Were the earth moving through a stationary ether, light traveling in a path 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. Bibliography: Word Count: 1274
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