![]() ![]() A holographic image is produced by constructive and destructive interference of a split laser beam. Holograms take advantage of the wave properties of light, as opposed to traditional photography which is based on geometric optics. A viewer can move around the image and see it from different perspectives. The name hologram means entire picture (from the Greek holo-, as in holistic), because the image is three-dimensional. Laser beams are used to produce holograms.The time it takes for the light to make the round trip can be used to make precise calculations of the Earth-Moon distance. Lasers are bounced off reflectors that astronauts left on the Moon.Lasers are used in industry to cut steel and other metals.Some barcode scanners use a laser beam.CDs are read by interpreting variations in reflections of a laser beam from the surface. Those audio and data-storage devices began replacing cassette tapes during the 1990s. This chapter began with a picture of a compact disc (see Figure 17.1).These properties lead to a number of applications in industry and medicine. Laser beams are directional, very intense, and narrow (only about 0.5 mm in diameter). ![]() You get the word laser (see Figure 17.2 (a)), which is the name of the device that produces such a beam of light. Take the first letters of all the words (except by and “of”) and write them in order. Some four decades later, Einstein’s idea found application in a process called, light amplification by stimulated emission of radiation. Such a light stream is said to be coherent. Those photons could go on to hit other excited atoms, and soon you would have a stream of in-phase photons. The exciting part is that you would have two photons with the same energy and they would be in phase. He suggested that the atom would emit a photon with that amount of energy, and it would be accompanied by the original photon. He considered an atom excited by a certain amount of energy and what would happen if that atom were hit by a photon with the same amount of energy. In 1917, Albert Einstein was thinking about photons and excited atoms. ![]() There will always be some blurring of images, no matter what the size of the aperture or the wavelength of light used to make an image. ![]()
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