Lecture #15 - Chemistry 2A-c W2009(Ch8-22)118 THE...

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1 THE PHOTOELECTRIC EFFECT light electron metal KE of electron Frequency of light ( ν ) 0 When v < v 0 , no electrons are ejected at any light intensity. KE of the ejected electrons depends only on the light’s frequency (v) When v > v 0 , the number of electrons is proportional to the light intensity. This lead Einstein to use Planck’s idea of quanta and postulate: “Electromagnetic radiation can be viewed as a stream of particle-like units called photons .” Kinetic energy =KE = (½)m e v 2 = h ν -h ν o = E – E 0 Chemistry 2A Chemistry 2A -c W2009 c W2009 (Ch8 (Ch8 -22)118 22)118
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2 At the beginning of the 20th century: Matter : Particle nature Electromagnetic radiation : Wave nature The two were thought to be quite separate in classical physics CLASSICAL PHYSICS Chemistry 2A Chemistry 2A -c W2009 c W2009 (Ch8 (Ch8 -23)119 23)119
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3 EINSTEIN’S THEORY OF RELATIVITY Einstein’s equation where m is the relativistic mass m m uc = 0 22 1 / m 0 is the rest mass The photon has m 0 =0. We have E=pc =h ν or p= h ν /c =h/ λ That is, photon has a momentum. E 2 = (pc) 2 + (m o c 2 ) 2 p = momentum= mu , u = velocity Chemistry 2A Chemistry 2A -c W2009 c W2009 (Ch8 (Ch8 -24)120 24)120 E = mc 2
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4 Compton collided X rays with electrons X rays electron λ h p = For a photon momentum Path of the electron is deflected: x-ray behaves as a particle The experiment shows that photons have momentum…. Chemistry 2A Chemistry 2A -c W2009 c W2009 (Ch8 (Ch8 -25)121 25)121 Compton effect Conservation of momentum
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5 de Broglie posed the question: “If light energy has particle-like properties, does matter have wave-like properties?” For a photon For a particle with mass m moving at u λ h p = h m = u p h m h = = u WAVE-PARTICLE DUALITY de Broglie postulated: So that Chemistry 2A Chemistry 2A -c W2009 c W2009 (Ch8 (Ch8 -26)122 26)122
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6 θ electron gun electrometer- detects electrons as current angle (q) current constructive interference destructive interference Davisson
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This note was uploaded on 02/25/2009 for the course CHEM chem 2a taught by Professor Ng during the Winter '09 term at UC Davis.

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Lecture #15 - Chemistry 2A-c W2009(Ch8-22)118 THE...

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