Major Concepts of Physics PHY102

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• Major Concepts of Physics PHY102

Lecture 17 Quantum mechanics (II) March 25th
Spring 2009 Prof. Liviu Movileanu http//physics.
syr.edu/courses/PHY102.07Spring/index.html
lmovilea_at_physics.syr.edu Room 211, Physics
Bldg., 443-8078
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Major Concepts of Physics PHY 102 Lecture 17
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Lecture objectives

• Review the Bohr model of the hydrogen atom
• 2. Excitations/Emissions/Ionizations
• 3. Fluorescence, Phosphorescence, and
  Chemiluminescence
• 4. The wave-particle duality
• 5. Matter waves. De Broglie hypothesis
• Experimental demo (The diffraction of electrons)
• 6. Conceptual example Electron diffraction
  experiment

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The hydrogen atom
                       
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The hydrogen atom
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• Ionization
• At n infinity, the energy is zero, and the
  electron is freed from the proton. 
• The ionization energy (also called the binding
  energy) is defined as the energy to liberate the
  electron, if the system is initially in its
  ground state.
• So, for hydrogen, the ionization energy is  
• E (at infinity) - E1 0 (-13.6) eV 13.6
  eV.

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How do we make Polarized Light?
II. Reflection -
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The Photoelectric Effect

• The phenomenon that when light shines on a metal
  surface, electrons are emitted

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The Bohr model of the hydrogen atom

• The Bohr model of hydrogen atom (1913) was the
  first step, in understanding the unique
  properties of the energy values for atoms.
• It explained the frequencies of light, emitted by
  hydrogen.
• Assumptions
• Electron circles the proton. But, only certain
  discrete values of the radius r are allowed.
• Each orbit forms a state.
• Bohrs Key Postulate
• These states satisfy
• L nh/2p, where n is an integer.
• n 1, 2, 3, .etc.
• Here, L angular momentum mvr for a circle.
  v speed of electron, r orbit radius.

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A freshly laundered blouse

Natural light
Ultraviolet light
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Electron microscope permits a magnification of
50,000
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Double-slit interference pattern

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  Matter Waves The Wave Nature of Matter Louis
de Broglie (1923) proposed that particles also
have duality. They can behave like a wave, or
like a particle. He proposed All particles also
have a wavelength ?. This wavelength displays
the wave-aspects. He postulated that the value of
the wavelength is ? h/mv. Here, v the
particle speed. Can also re-write this relation
as ? h/p. Here, p is defined by p mv,
and is called the momentum of the particle.  

•  
•  

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A double-slit experiment
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A double-slit experiment
 
                       
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A double-slit experiment
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Experimental Confirmation Electron
diffraction was observed in 1925 by Davisson and
Germer. A crystal served to diffract the
electrons. No explanation, but this Matter waves
exist! Do electron diffraction demo. Later,
precise experiments confirmed the de Broglie
relation for ?. Conclusion Duality
holds for both matter and radiation.  
 
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The Davisson-Germer experimental set-up
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Electron diffraction pattern from a
polycrystalline Aluminum sample
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X-ray diffraction pattern from the same sample
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Conceptual example
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Conceptual example
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Announcements

• Reading Chapters 27th, 28th
• Section 27.7 The Bohr model of the hydrogen
  atom atomic energy levels, pp. 1002-1004 (Not
  conceptual example 27.8!!)
• Section 28.1 The wave-particle duality
• Section 28.2 Matter waves pages 1016-1020 (Not
  conceptual example 28.2!!)
• 2. Homework. HW 7 is due on the next weeks lab

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Quiz 5

• What is the energy of photons, if their
  wavelength is 515 nm?
• What is the color of light in Question 1?
• The work function of potassium is 2 eV. 1 eV1.6
  10-19 J. Blue light with a wavelength of 400
  nm is shined on a potassium plate.
• What is the energy of the incident photons?
• Are the electrons ejected off?
• If so, what is their kinetic energy in Joules?

1 nm10-9 m h6.62610-34 Js c3108 ms-1 1
eV1.610-19 J
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