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This file contains a copy of one paper and an excerpt from a book: J. Galang, A. Restelli, E. W. Hagley and C. W. Clark, “A Green Laser Pointer Hazard,” NIST Technical Note 1668 (National Institute of Standards and Technology, Gaithersburg, MD 2010). As a work product of the U.S. Government, this document is not subject to copyright. C. Lanczos, The Einstein Decade (1905-1915) (Paul Elek (Scientific Books ) Ltd., London 1971) pp. 62-67. This is an extract from a work that a
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   This file contains a copy of one paper and an excerpt from a book: J. Galang, A. Restelli, E. W. Hagley and C. W. Clark, “ A Green Laser Pointer Hazard, ”  NIST Technical Note 1668 (National Institute of Standards and Technology, Gaithersburg, MD 2010). As a work product of the U.S. Government, this document is not subject to copyright. C. Lanczos, The Einstein Decade (1905-1915)  (Paul Elek (Scientific Books ) Ltd., London 1971)  pp. 62-67. This is an extract from a work that appears to be in the public domain, and is freely available at http://archive.org/details/TheEinsteinDecade1905-1915 . The paper is a convenient and accessible reference for understanding the operating principles of the familiar green laser pointer. These demonstrate some points germane to Einstein ’ s 1905  photoelectric effect paper contained in the file “ The photoelectric effect ”  in our course collection of srcinal scientific literature: 1) The operation of Stokes ’  law in the primary pumping process (Fig. 1): a photon with a wavelength of 808 nm is absorbed and causes the emission of a photon of wavelength 1064 nm. This corresponds to the second equation in sec. 7 of Einstein ’ s paper:  ν 2   ≤    ν 1  2) Application of the equation  E = hν  in the context of photon conversion. Two photons of wavelength λ  1  = 1064 nm are converted to a single photon of wavelength λ  2  in an energy-conserving process. Thus  E = 2   hν 1  = 2 hc / λ  1  = hc / λ  2  , where c  is the speed of light. Thus λ  2  = 532 nm, which is the wavelength of the green laser pointer. It may be confusing to some why we always identify light by its wavelength rather than its frequency. Fig. 6 provides an example of the historical reason for this. Even in Einstein ’ s day it was easy to measure wavelengths of 532 nm by diffraction grating spectrometers, whereas the direct measurement of optical frequencies (5.64 x 10 14  Hz in this case) has become possible only recently. Lanczos ’  extract provides a lucid and accessible summary of “ a miracle which has hardly been matched in the history of science ” : Einstein ’ s 1917 derivation of Planck  ’ s radiation formula. In this work, Einstein identified the process of stimulated emission that underlies the principle of laser action. An English-language translation of Einstein ’ s 1917 paper is available free on Google Books.    NIST Technical Note 1668 A Green Laser Pointer Hazard Jemellie Galang Alessandro Restelli Edward W. Hagley Charles W. Clark   NIST Technical Note 1668 A Green Laser Pointer Hazard Jemellie Galang Alessandro Restelli Edward W. Hagley Charles W. Clark  Electron and Optical Physics Division  Physics Laboratory July, 2010 U.S. Department of Commerce Gary Locke, Secretary  National Institute of Standards and Technology  Patrick D. Gallagher, Director   Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the  National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose. National Institute of Standards and Technology Technical NOTE 1668 Natl. Inst. Stand. Technol. Tech. Note 1668, 9 pages (July, 2010) CODEN: NTNOEF Note added April 2013: In April, 2013, the National Institute of Standards and Technology published specifications for a testbed for measuring laser pointer radiation: “ Accurate, inexpensive testing of laser pointer power for safe operation, ”  J. Hadler and M. Dowell,  Meas. Sci. Technol.   24 , 045202 (2013), doi:10.1088/0957-0233/24/4/045202 The test protocols described in that paper are recommended by NIST for accurate measurement of optical power emitted from handheld lasers.
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