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andersonChap2 - THE BEGINNINGS OF THE QUANT UM THEORY THE...

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THE BEGINNINGS OF THE QUANT UM THEORY THE PHOTOELECTRIC EFFECT S5 hv = W + T . A lthough the term " laser" can be used for all s uch devices employi ng pho tons, it.is co mmon practice to regard a device operatin g in the microwave region as a maser and one operating at optical f reque ncies as a laser. However, th is distinction cannot be held too rigidly, since it is possible to pum p at mic rowave frequencies an d to ampl ify at optica l frequencies, or vice versa. Since there are regi ons of the elec tromagnetic spect rum whe rein it is extremely difficult either to ob tain a mon ochromatic source or to a mpl ify an existing signal, it wou ld be attr act ive to devise lasers whic h cou ld opera te at these freq uencies b ut whi ch cou ld be pump ed at frequencies th at are readily a tta in- able (such as "white " light). T his goal provides i mpetus for a g rea t deal of research on the e nergy levels of ato mic, molecular and man y-body systems." 5. THE PARTICLE NA TURE OF PHOTONS Th e mea surem ent of th e char ge of the ele ctron by Mil lik an " in 1909 established the fact t hat charge, as well as energy, is qu antiz ed. That is, any a ccumu l atio n of c ha rge mu st consist of an integr al mu ltiple of the electronic charge. On ce the value of the el ect ron ic cha rge was kno wn, it was possible to det ermin e the ele ctron 's ma ss from th e va lue of el m ob tained by T homson-? and others. As a resu lt of a great deal of rese arch with cath ode ra y tu bes, a nu mbe r of inter esting demonstration s were devised to verify the particle nature of the electro n. The ele ctron 's mass is rn o = 9.109 x 10- 28 gram and its charge is e = 4.803 x 10- 10 stat c oulomb = 1.602 x 10- 19 cou lom b. It is easy to accept the particle nature of the elec tron because we can define its mass, we ca n acc elerat e it, and we can mak e it b eha ve as we think a pa rticle ough t to beha ve. A photon , on the other hand , ha s no rest mass and canno t be accelerate d; howe ver, we ha ve seen that it has momentum associated with it in bot h classical electr omag netic th eory and in the special theory of rel ati vity (see Equation 1.18). W e will now discuss two very important exper imental event s which can be best explained by assuming tha t th e elec tron interacts with a single photon as if the photon were a localized part icle r ather than a wave fr ont. Thu s, th e qua ntum or p articl e natur e of lig ht dominates its wave na tur e in these exp eriment s. The first is th e well-known photoelectric effect an d the second is th e Comp ton effect. 6. THE PHOTOELECTRIC EF FE CT Ligh t incid e nt up on a metal sur face can, und er some c ondi tions, eject el ectrons fr om the surface. The se electrons are called ph ot oelectr ons, not tha t the y diff er from other electrons, but merel y to ide ntify the ir source. The following facts mu st be explained by a satisfactory the ory of the ph otoelectric 8 B. Lengyel, Introduction to Laser Physics. J ohn Wiley and Sons, In c. New Y ork , 1966. See also, Lasers and Light, Reading sfr om Scimufi« American. W . H. Free
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