Class21HO - Class 21 Early Quantum Mechanics and the Wave...

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Class 21 Early Quantum Mechanics and the Wave Nature of Matter Physics 106 Fall 2011 Press CTRL-L to view as a slide show.
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Last Time Last time we discussed: I Optical systems I Midterm 2
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Learning Outcomes Today we will discuss: I Quick Overview of Quantum Mechanics I Blackbody Radiation I Photoelectric Effect I X-ray diffraction I Compton scattering
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Quantum Mechanics - An Overview
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April 27, 1900 In a speech before the Royal Institution, Lord Kelvin proposed that we basically understood all of physics, except for "two dark clouds on the hoirizon." These were I The null result of the Michelson-Morley experiment I The inability of classical physics to explain blackbody radiation
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Need for Quantum Physics Blackbody Radiation I The electromagnetic radiation emitted by a heated object Photoelectric Effect I Emission of electrons by an illuminated metal Spectral Lines I Emission of sharp spectral lines by gas atoms in an electric discharge tube
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Blackbody Radiation Light is emitted from hot objects in bundles (quanta) of energy
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Blackbody Radiation I An object at any temperature emits electromagnetic radiation I The spectrum of the radiation depends on the temperature and properties of the object
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Blackbody Radiation Graph I As T increases, the total energy emitted increases I As T increases, the peak of the distribution shifts to shorter wavelengths
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Wien’s Law I Based on thermodynamics and EM Theory I Gave good agreement at short wavelengths
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Wien’s Law I But it wasn’t very good at long wavelength
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Rayleigh-Jeans Law I Also based on thermodynamics and EM Theory I Gave good agreement at long wavelength
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Rayleigh-Jeans Law I But it didn’t agree at all at short wavelength I This was called the "ultraviolet catastrophe"
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Blackbody Radiation I Classical physics gave two laws that didn’t agree I . . . and neither agreed with all the data
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Max Planck I 1858 -1947 I Introduced a "quantum of action," h
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Planck’s Resolution I Oscillators in matter could only emit light at discrete energies I E n = nhf I n is called the quantum number I f is the frequency of oscillation I h is Planck’s constant
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Planck’s Constant There are many versions I h = 6 . 626 × 10 - 34 Js (SI) I h = 4 . 136 × 10 - 15 eV s (eV) I hc = 1240 eV nm I ~ = h / 2 π ("hbar")
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Today’s Interpretation I Light from atoms in the blackbody is emitted as photons from atoms I Each photon of light carries energy E = hf
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Conclusion 1 I At very small scales, measurements of many physical quantities can only have discrete values. These include I Energy of bound electrons I Angular momentum I Angular momentum component along one axis I Spin
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Photoelectric Effect Light is absorbed by matter in quanta of energy called photons
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