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Unformatted text preview: 1 BACKGROUND QUANTUM PHYSICS 19001925 In these notes we briefly discuss several of the important developments in quantum physics in the period 19001925 before the invention of quantum mechanics by Heisenberg and Schrodinger [1, 2]. Blackbody radiation (Planck 1900) Consider the electromagnetic radiation inside a cavity with walls at temperature T , see Fig.1 . The energy is distributed across the frequency spectrum. Let U ( ) d be the energy of radiation between frequency and + d . Based on thermodynamics Wien predicted U ( ) = 3 f ( /T ) where f is an unknown function. ( ( ) I U Q Q , v FIG. 1. The intensity of light emitted at frequency is proportional to the corresponding energy density per unit frequency interval U ( ) in the cavity of volume V . Using classical elecromagnetic theory Rayleigh and Jeans found U ( ) = (8 V/c 3 ) 2 kT ; at high frequencies this diverges as does the total energy integraltext U ( ) d . Wien fitted the measured distribution to the function U ( ) 3 e p/ ( kT ) where k is Boltzmanns constant and p is a numerically fitted parameter, though the fit was not very good. Planck obtained a perfect fit, within experimental error, to the measured spectrum with the function U ( ) = 8 V c 3 h 3 1 e h/ ( kT ) 1 . The fitting parameter h = 6 . 6 10 34 Js is Plancks constant. (Note: h = planckover2pi1 where planckover2pi1 = h/ (2 ) and = 2 .) Note that Plancks law agrees with Wiens law. Moreover it agrees with classical physics ( in the guise of the RayleighJeans law ) when kT >> h (check it). Planck introduced the revolutionary quantum hypothesis that energy is emitted(check it)....
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This note was uploaded on 01/22/2012 for the course PHYS 3143 taught by Professor Kennedy during the Fall '10 term at Georgia Institute of Technology.
 Fall '10
 Kennedy
 mechanics, Quantum Physics, Radiation

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