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3/21/2013
Quantum Physics versus Classical Physics
The Thirty-Year War (1900-1930)
Models of the Atom
• Democritus, Dalton and Mendeleïev
Interactions between Matter and Radiation
• 1897 Thomson’s Plum Pudding Model and the Discovery of the Electron
• Models of the Atom
• 1911 Rutherford’s Model and the Discovery of the Nucleus
• Bohr’s Model of the Atom
• 1913 Bohr’s Planetary Model and Spectral Lines
• Planck’s Blackbody Radiation
• 1926 Schrödinger’s Cloud Model and the Probability Wave Function
• Einstein’s Photoelectric Effect
• Compton’s Effect
• De Broglie’s Matter Waves
• Quantum Mechanics
Democritus, Dalton, and Mendeleev
1897 Thomson’s Plum Pudding Model
and the Discovery of the Electron
Dalton’s compounds
Mendeleev's periodic table of elements
Experimental Set-Up
Plum Pudding Model
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1911 Rutherford’s Model
and the Discovery of the Nucleus
Bohr’s Model of the Atom
• Classical Model of the Atom
• Atomic Spectra
• 1913 Bohr’s Planetary Model
• Bohr’s Atom
• Atomic Spectra Explained
• Bohr’s Correspondence Principle
• Energy-Level Diagrams
Classical Model of Atom
Atomic Spectra
Why do atoms of a given element exhibit only certain spectral lines?
Why do atoms absorb only the frequencies (wavelengths) that they
were emitting?
Hydrogen Spectral Lines
The Balmer Spectrum of Hydrogen
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1913 Bohr’s Planetary Model
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What is the arrangement of the electrons around the nucleus?
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What keeps the electron from falling into a positive nucleus by
electrical attraction?
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Why do elements exhibit different atomic spectra of discrete lines?
Atomic Spectra Explained
1900 Planck’s Blackbody Radiation
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Thermal Radiation
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Blackbody
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Blackbody Radiation
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The UV Catastrophe
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Planck’s Quantum of Energy
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Blackbody Explained by Bohr’s Atom
Bohr’s Atom
Energy-Level Diagrams
Thermal Radiation
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Blackbodies
Blackbody Radiation
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Stars’ Surface Temperatures
Power (area under curve) increases with T
Stefan-Boltzmann’s Law
P =  A e T4
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Peak wavelength decreases as T
increases
Wien’s Law
max T = 0.002898 m K
The energy output and peak wavelength of the blackbody
radiation curve both give the surface temperature of stars.
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The UV Catastrophe
Formative Quiz
The temperature of your skin is approximately 35 0C.
(a) What is the peak wavelength of the radiation it emits?
(b) What is the total power emitted by your skin. Assume that the
area of your skin is 2.0 m2.
(c) Why don’t you glow as bright as a light bulb?
Blackbody Explained by Bohr’s Atom
Our Universe is a Blackbody
(Cosmology and Quantum Physics)
1905 Einstein’s Photoelectric Effect
Albert Einstein
Nobel Prize 1921
• Experiment
• Classical Physics
• Experimental Results
• Einstein’s Interpretation
• Applications
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Experiment
Einstein’s Interpretation
Kmax = h f - 
Kmax maximum kinetic energy of elected electrons (photoelectrons)
h Planck’s constant
f frequency of light
 work function of the metal
Applications
Classical Physics vs Experimental Results
Formative Quiz
The maximum electron energy in a photoelectric experiment is 3.4
eV. When the wavelength of the illuminating radiation is
increased by 25%, the maximum electron energy drops to 2.6 eV.
(a) What is the original wavelength of the illuminating radiation?
(b) What is the work function of the emitting surface?
Medicine
Automatic Door Openers
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Film
Photomultiplier Tube
Automatic Camera
Solar Panels
Smoke Detector
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Experiment
1923 Compton’s Effect
Arthur Compton
Nobel Prize 1927
• Experiment
• Classical Physics
• Experimental Results
• Compton’s Interpretation
• Applications
Classical Physics vs Experimental Results
Compton’s Interpretation
Photons have momentum p = h / 
Compton Shift  = ’ - 0 = (h / (me c)) (1 – cos )
Compton’s Wavelength of Electron C = h / (me c) = 0.00243 nm
Peak at 0 Photons interact with electrons tightly bound to the atom
(effectively they collide with the atom itself) leading to a Compton shift
too small to be detected.
Formative Quiz
X-rays of wavelength  = 22 pm are scattered from a carbon target
and the scattered x-rays are detected at 850 to the incident beam.
Application
Dental X - Rays
(a) What is the Compton shift of the scattered x-rays?
(b) What percentage of the initial x-ray energy is transferred to an
electron in such scattering?
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Photons and Electromagnetic Waves
1923 De Broglie’s Matter Waves
Louis De Broglie
Nobel Prize 1929
Shortest doctoral thesis on record
• Matter Waves
• Davisson-Germer Experiment
• Electron Diffraction and Interference
Patterns
• Application: The Electron Microscope
Matter Waves
1927 Davisson-Germer Experiment
De Broglie’s wavelength
 = h / p = h / (m v)
frequency of a particle
f = E/h
Equations contain both particle (p and E) and wave ( and f) quantities.
First experimental evidence of the wave nature of particles (electrons).
Electron Diffraction Pattern
The Electron Microscope
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Quantum Mechanics
• 1925 Heisenberg’s Uncertainty Principle
• Barrier Tunnelling
• Applications
Head of an Antarctic Mite
magnified 1500 times
• 1926 Schrödinger’s Cloud Model
• Probability Density and Electron’s Orbitals
• Schrödinger's Cat
• 1928 Dirac’s Equation
3-D nanostructure: “flowers"
of silicon carbide and gallium
Hydrothermal Worm
1925 Heisenberg’s Uncertainty Principle
Barrier Tunnelling
x px  h / 4
E t  h / 4
Applications
Scanning Tunnelling Microscope
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Alpha Decay
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Nuclear Fusion
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Scanning Tunnelling Microscope
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1926 Schrödinger’s Cloud Model
Schrödinger’s Cat
Probability Density and Electron’s Orbitals
1928 Dirac’s Equation
• Dirac’s equation describes the evolution of the wave function of a
relativistic particle. It includes quantization of energy.
• Dirac predicted the existence of antimatter (positron). The positron
was first observed in 1933.
First positron track observed in a cloud chamber.
Summary
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1897 Thomson Discovery of the Electron
1900 Planck Blackbody Radiation
1905 Einstein Photoelectric Effect
1911 Rutherford Discovery of Nucleus
1913 Bohr Quantum Model ofAtom
1923 Compton Effect
1923 De Broglie Matter Waves
1925 Heisenberg Uncertainty Principle
1926 Shrödinger Wave Function
1927 Davisson-Germer Experiment
1928 Dirac Equation
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