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Quantum Physics…
the world is about to get even
weirder!
Just when everything seemed to be
working…
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Reading: Moore; Unit Q: chps 1-4
Lord Kelvin’s two small dark
clouds:
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Michelson-Morley Experiment
Blackbody radiation
Planck’s Radiation Law
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Light (energy) is
quantized … E = hf
Blackbody spectrum
explained!
Working with quanta…
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Energy carried by light…
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En = nhf
Energy quantization of a spring
Wien’s Law and the Stefan-Boltzmann Law
Light does not behave quite like a classical
wave!
…it gets weirder!
The Photoelectric Effect
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Evidence that light behaves like a particle
(sometimes)!
Enter the photon
Concept of the Work Function…
Ek = hf - f
When waves act like particles!
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The strange way in
which photons
interfere with
themselves!
Compton Scattering
Light – is it:
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Wave
Particle
Both
Neither?
f
q
h
 
1  cosq 
mc
When Particles act Like Waves!
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Prince Louis de Broglie
makes a bold prediction
The de Broglie wavelength:
h

p
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Confirmed by Davisson and
Germer 1927
Particles, Waves and Quantons
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Particle and Wave are
macro-world concepts
“Quanton” is the
quantum world actor
The Great Heresy!
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A wave “is a particle”
- A particle “is a
wave”
The Schroedinger
Equation and
Heisenberg’s Matrix
Mechanics...
...a fundamental
blurring of the
universe
It looks like Heisenberg - I
think, I’m pretty sure, I’m not certain...
The Uncertainty
Principle
h
xp 
4
h
E t 
4
The Copenhagen Interpretation...
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QM is a complete theory
that tells us that the
world, at the quantum
level, is governed by
statistical law. It rules
out “classical” or “naïve”
realist views of nature.
As an example, consider
the following applet
demonstrating the
Hydrogen atom.
The Bohr Atom
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What do you do when theories fail?
Our understanding of atoms and atomic
physics circa 1910
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currant buns, gold foil, “saturnalia” and
others...
Quantization
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the Nicholson atom
Bohr
applet demonstrating the Hydrogen atom.
“as soon as I saw Balmer’s formula it all became
clear to me”
1
1 1
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

 R 2  2 

2 n 
Bohr made two main postulates:
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an atom has a number of stable states in
which the electrons orbit the nucleus in
accordance with Newton’s Laws but do not
radiate energy
an atom emits or absorbs energy only
when an electron moves from one stable
state to another
Bohr made a further auxilliary assumption
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electrons moved in orbits for which the
orbital angular momentum was
quantized...
 h 
l  mvr  n 

 2 
In Bohr’s footsteps...
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forces
energy
momentum
putting it all together...
n h 0
rn 
 me2
2
2
4
me
En   2 2 2
8 0 h n
and so..
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We can recover Balmer’s formula:
En  Em  h  h
1
4
c

me
1 1
 2 3 ( 2  2)
 8 0 ch l n
taking stock...
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We can now:
determine the size of the H atom
 predict the spectrum of H
Can we extend this line of reasoning to other
atoms:
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