Lecture_03 - PHYS 342 Fall Fall 2010 Lecture 03: The Roots...

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PHYS 342 all 2010 Fall 2010 Lecture 03: The Roots of Quantum Theory: The “ roblem” Experiments Problem Experiments Blackbody Radiation Photoelectric Effect Discrete Line Spectra Bohr’s semiclassical model for H atom m mf m Compton Effect Michelson-Morley Experiment – more later Ron Reifenberger Birck Nanotechnology Center Purdue University Lecture 03 1
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Blackbody Radiation (1859/1879/1884) Combining Thermodynamics with E & M T Stefan-Boltzmann Law 4 r P eA T P r = power radiated by hot object (in Watts) e = emissivity of object; 0<e<1 = Stefan constant = 5.6703 x 10 -8 W/(m 2 K 4 ) A = area of emitting (hot) object T = temperature of hot object (in K) 2
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Summary: Blackbody Radiation - light emission from objects heated to a temperature T Important: “How does radiation interact with matter?” lackbody “Thermal” Radiation (from experiment): Blackbody Thermal Radiation (from experiment): • Continuous light emission with no well defined emission “lines” • Light spectrum (to first approximation) does not depend on material that is heated, only on absolute temperature T Energy density of radiation field is in equilibrium with temperature of heated object thermodynamics 3
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Spectral Distribution of Blackbody Radiation PHET, University Colorado at Boulder, see: http://phet.colorado.edu/simulations/sims.php?sim=Blackbody_Spectrum 4
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What’s going on? ) l t s i th h t lls ) E ilib i m is st blish d; l a) Electrons in the hot walls emit light at all frequencies b) Equilibrium is established; only certain light “modes” persist T E at surface is finite; damped mode . . . . . . . L y λ 1 λ 2 λ 3 λ 7 E at surface is zero; undamped mode . . . . . . . λ 4 λ 5 λ 6 ) he shorter the wavelength, the more modes can fit inside the cavity L x c) The shorter the wavelength, the more modes can fit inside the cavity c) Assume each persistent mode has same average energy = k B T 23 (1 . 3 8 1 0/ ) B kJ K  d) Finally, count ALL possible modes and sum them up to obtain the “spectral energy density” of the EM fields vs. wavelength 5
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Classical physics unable to explain shape of emitted light spectrum Classical prediction - radiation field inside cavity is comprised )/ m] V R visible of standing EM waves.
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This note was uploaded on 12/08/2011 for the course PHYS 342 taught by Professor Staff during the Fall '08 term at Purdue University.

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Lecture_03 - PHYS 342 Fall Fall 2010 Lecture 03: The Roots...

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