AP1200_Ch7_ModernPhysics

AP1200_Ch7_ModernPhysics - ModernPhysics...

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    Modern Physics Imagination is more important  than knowledge… Albert Einstein
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    History At the end of the 19 th  century, scientists believed that  they had learned almost everything about physics:  Newtonian mechanics Maxwell’s theories to unify electricity and  magnetism Laws of thermodynamics and kinetic theory But Classical physics fails to explain the behaviour of  matter on the atomic scale:  Blackbody radiation Photoelectric effect Emission of specific spectral lines from atoms
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    Development  Major revolutions of physics in early 20 th   century: Einstein developed his  special theory of relativity Planck provided the basic ideas that led to the  birth of  quantum theory   New theories and their applications have led  to many important achievements Evolution goes on…
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    The Blackbody Radiation In 1900 Planck discovered a formula for black radiation  that was  in complete agreement with experiment data  at  all wavelength.   ) 1 ( 2 ) , ( / 5 2 - = T k hc B e hc T I λ π Quantization of energy: nhf E n = Plank made two assumptions:  (a) the molecules can have  only discrete values of energy   (b) the molecules emit or  absorb energy in discrete packets called photons
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    Wave Particle Duality Waves as particles Photo electric effect Compton scattering Pair production Particles as waves de Broglie  wavelength Electron diffraction mC 2    =   E E    =  hf
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    The Photo Electric Effect Waves as Particles What is happening here? 
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    Observations I I V - V s f K max P Zn f c Photoemission occurs   instantaneously . No  photoelectrons are emitted  below a certain frequency. Cutoff frequency:  f c  = c/ λ c   Work function:   φ  = hf c    The photoelectron I can  be stopped at a negative  stopping potential V s   e V s  = KE max   
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    Observations II The number of photoelectrons emitted is proportional to  the light intensity, but independent of the frequency. The maximum KE of a photoelectron is proportional to  the incident frequency, but independent of the intensity φ - = hf K max - = hf V e s The energy of the photon E=hf The maximum KE of a photoelectron: The stopping potential: 
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    The Compton Scattering Waves as Particles What is happening here?  Photon transfers part of  its energy and  momentum to electron Increase in wavelength Decrease in frequency
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    The Compton Scattering ( 29 θ λ cos 1 0 - = - c m h e Wavelength shift:
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    Pair Production Waves as Particles What is happening here? 
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    ( ( wave wave ) ) λ λ dB dB = = h /p h /p ( ( particle particle ) ) De Broglie Wavelength Particles as Waves De Broglie stated that all matter has a wave-
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This note was uploaded on 01/11/2011 for the course AP 1200 taught by Professor Michela.vanhove during the Spring '10 term at City University of Hong Kong.

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AP1200_Ch7_ModernPhysics - ModernPhysics...

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