AndersonChap2 - THE BEGINNINGS OF THE QUANT UM THEORY THE PHOTOELECTRIC EFFECT S5 Although th e term laser can be used for all such devices employi

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THE BEGINNINGS OF THE QUANTUM THEORY THE PHOTOELECTRIC EFFECT S5 hv = W + T . Although the term "laser" can be used for all such devices employi ng pho tons, common practice to regard a device operating in the microwave region as a maser and one operating at optical frequencies as a laser. However, th is distinction cannot be held too rigidly, since it is possible to pum p at microwave frequencies an d to amplify at optical frequencies, or vice versa. Since there are regi ons of the electromagnetic spectrum wherein it is extremely difficult either to obtain a mon ochromatic source or to amplify an existing signal, it would be attractive to devise lasers which could operate at these frequencies but which could be pumped at frequencies th at are readily attain- able (such as "white" light). T his goal provides impetus for a great deal of research on the energy levels of ato mic, molecular and man y-body systems." 5. THE PARTICLE NATURE OF PHOTONS The measurement of the charge of the electron by Millikan" in 1909 established the fact that charge, as well as energy, is quantized. That is, any accumulation of charge mu st consist of an integral mu ltiple of the electronic charge. Once the value of the electronic charge was known, it was possible to determine the electron's mass from th e value of el m ob tained by T homson-?and others. As a result of a great deal of research with cathode ra y tu bes, a number of interesting demonstrations were devised to verify the particle nature of the electro n. The electron's mass is rn o = 9.109 x 10- 28 gram and its charge is e = 4.803 x 10- 10 statcoulomb = 1.602 x 10- 19 coulomb. It is easy to accept the particle nature ofthe electron because we can define its mass, we can accelerate it, and we can make it behave as we think a particle ough t to behave. A photon, on the other hand, has no rest mass and cannot be accelerated; however, we have seen that it has momentum associated with it in both classical electromagnetic theory and in the special theory of relativity (see Equation 1.18). W e will now discuss two very important experimental events which can be best explained by assuming that the electron interacts with a single photon as if the photon were a localized particle rather than a wave front. Thu s, th e quantum or particle nature of light dominates its wave nature in these experiments. The first is th e well-known photoelectric effect an d the second is th e Compton effect. 6. T HE PHOTOELECTRIC EFFECT Light incident upon a metal surface can, under some conditions, eject electrons from the surface. These electrons are called photoelectrons, not that they differ from other electrons, but merel y to ide ntify their source. The following facts mu st be explained by a satisfactory theory of the photoelectric 8 B. Lengyel, Introduction to Laser Physics. J ohn Wiley and Sons, Inc. New York, 1966. See also, Lasers and Light, Readingsfrom Scimufi« American. W . H. Freeman an d Co., San Francisco, 1969.
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This note was uploaded on 08/06/2008 for the course PHYS 25 taught by Professor Nelson during the Spring '08 term at UCSB.

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AndersonChap2 - THE BEGINNINGS OF THE QUANT UM THEORY THE PHOTOELECTRIC EFFECT S5 Although th e term laser can be used for all such devices employi

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