Quantum Physics

924 Chapter 28 Atomic PhysicsM 2M 1LaserL 2BL 1FilmCourtesy of Central Scientific Company(a)(b)Figure 28.22 (a) Experimental arrangement for producing a hologram. (b) Photograph of a hologrammade with a cylindrical film. Note the detail of the Volkswagen image.APPLICATIONLaser TechnologySince the development of the first laser in 1960, laser technology has exhibitedtremendous growth. Lasers that cover wavelengths in the infrared, visible, andultraviolet regions of the spectrum are now available. Applications include thesurgical “welding” of detached retinas, “lasik” surgery, precision surveying andlength measurement, a potential source for inducing nuclear fusion reactions,precision cutting of metals and other materials, and telephone communicationalong optical fibers. These and other applications are possible because of theunique characteristics of laser light. In addition to being highly monochromaticand coherent, laser light is also highly directional and can be sharply focused toproduce regions of extremely intense light energy.Scientist checking the performanceof an experimental laser-cuttingdevice mounted on a robot arm. Thelaser is being used to cut through ametal plate.APPLICATIONHolographyPhilippe Plailly/Photo Researchers, Inc.HolographyOne interesting application of the laser is holography: the production of threedimensionalimages of objects. Figure 28.22a shows how a hologram is made.Light from the laser is split into two parts by a half-silvered mirror at B. One partof the beam reflects off the object to be photographed and strikes an ordinaryphotographic film. The other half of the beam is diverged by lens L 2 , reflects frommirrors M 1 and M 2 , and finally strikes the film. The two beams overlap to form anextremely complicated interference pattern on the film, one that can be producedonly if the phase relationship of the waves is constant throughout the exposure ofthe film. This condition is met through the use of light from a laser, because suchlight is coherent. The hologram records not only the intensity of the light scatteredfrom the object (as in a conventional photograph), but also the phase differencebetween the reference beam and the beam scattered from the object. Becauseof this phase difference, an interference pattern is formed that produces animage with full three-dimensional perspective.A hologram is best viewed by allowing coherent light to pass through the developedfilm while you look back along the direction from which the beam comes.Figure 28.22b is a photograph of a hologram made using a cylindrical film.28.13 ENERGY BANDS IN SOLIDSIn this section we trace the changes that occur in the discrete energy levels ofisolated atoms when the atoms group together and form a solid. We find that insolids, the discrete levels of isolated atoms broaden into allowed energy bands