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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 Mendeleïev
1897 Thomson’s Plum Pudding Model
and the Discovery of the Electron
Josepf James Thomson
Nobel Prize 1906
Democritus
ca. 460 BC – ca. 370 BC
John Dalton
1766-1844
Experimental Set-Up
Mendeleïev
1834-1907
Plum Pudding Model
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Bohr’s Model of the Atom
1911 Rutherford’s Model
and the Discovery of the Nucleus
• Classical Model of the Atom
• Atomic Spectra
Ernest Rutherford
• 1913 Bohr’s Planetary Model
Nobel Prize 1908
(Chemistry)
• Bohr’s Atom
• Atomic Spectra Explained
• Bohr’s Correspondence Principle
• Energy-Level Diagrams
Classical Model of Atom
Atomic Spectra
absorption spectra dark lines
emission spectra bright lines
Why do atoms of a given element exhibit only certain spectral lines?
Why do atoms absorb only the frequencies (wavelengths) that they were
emitting?
Why is the number of lines of the emission spectrum not always equal to
the number of lines of the absorption spectrum?
1913 Bohr’s Planetary Model
Niels Bohr
Nobel Prize 1922
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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?
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Atomic Spectra Explained
Bohr’s Atom
1900 Planck’s Blackbody Radiation
Energy-Level Diagrams
Max Planck
Nobel Prize 1918
Thermal 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
Blackbody
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Blackbody Radiation
Formative Quiz
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Power (area under curve) increases with T
Stefan’s Law
P=AeT
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Peak wavelength decreases as T increases
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 lightbulb?
Wien’s Law
max T = 0.002898 m K
Blackbody Explained by Bohr’s Atom
The UV Catastrophe
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
1923 Compton’s Effect
Arthur Compton
Nobel Prize 1927
Smoke Detector
• Experiment
• Classical Physics
• Experimental Results
• Compton’s Interpretation
• Applications
Experiment
Classical Physics vs Experimental Results
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Compton’s Interpretation
Formative Quiz
Photons have momentum p = h / 
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.
(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?
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.
Application
Photons and Electromagnetic Waves
Dental X - Rays
Matter Waves
1923 De Broglie’s Matter Waves
Shortest doctoral thesis on record
De Broglie’s wavelength
 = h / p = h / (m v)
Louis De Broglie
Nobel Prize 1929
• Matter Waves
• Davisson-Germer Experiment
• Electron Diffraction and Interference
Patterns
frequency of a particle
f = E/h
Equations contain both particle (p and E) and wave ( and f) quantities.
• Application: The Electron Microscope
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Electron Diffraction Pattern
1927 Davisson-Germer Experiment
First experimental evidence of the wave nature of particles (electrons).
The Electron Microscope
Quantum Mechanics
• 1925 Heisenberg’s Uncertainty Principle
• Barrier Tunnelling
• Applications
Penguin Feather
magnified 1500 times
• 1926 Schrödinger’s Cloud Model and the Probability Wave Function
• Probability Density and Electron’s Orbitals
• Schrödinger's Cat
• 1928 Dirac’s Equation
Head of an Antarctic Mite
magnified 1500 times
1925 Heisenberg’s Uncertainty Principle
Barrier Tunnelling
Werner
Heisenberg
Nobel Prize 1932
x px  h / 4
E t  h / 4
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Applications
Scanning Tunnelling Microscope
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Alpha Decay
•
Nuclear Fusion
•
Scanning Tunnelling Microscope
Probability Density and Electron’s Orbitals
1926 Schrödinger’s Cloud Model and the
Probability Wave Function
Erwin Schrödinger
Nobel Prize 1933
Schrödinger’s Cat
1928 Dirac’s Equation
• Dirac’s equation describes the evolution of the
wave function of a relativistic particle. It
includes quantization of energy.
Paul Dirac
Nobel Prize 1933
• Dirac predicted the existence of antimatter
(positron). The positron was first observed in
1933.
First positron track observed in a cloud chamber.
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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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