Quantum Continued

1
Quantum Continued
PH103
Dr. James van Howe Lecture 19
May 2, 2008
Electron diffraction patterns
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Which athlete holds the record number of Olympic
medals?

• Jenny Thompson (swimming)
• Carl Lewis (Track)
• Larisa Latynina (Gymnastics)
• Alexandr Karelin (Greco-Roman Wrestling)

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True or False
A particle of light has zero rest energy
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True or False
Increasing the intensity of photons incident on a
metal surface increases the kinetic energy with
which electrons are ejected from the metal.
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True or False
Knowing that Ehf, for black body radiation, as
frequency goes up indefinitely, so does the
intensity
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The color of a photon that scatters off of an
electron (Compton scattering) becomes ______
compared to before scattering

• Bluer
• Redder
• Does not change color

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As the momentum of a particle gets smaller, its
de Broglie wavelength becomes

• Larger
• Smaller
• Stays the same

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Last Time Blackbody Radiation
Classical

• Every mode has same energy kT
• Every mode has same probability
• As mode (frequency) goes to infinity, so does
  intensity

Black Body curve for T 293 K
Quantum

• Each mode has energy Ehf light is quantized
  into photons
• As mode (frequency) goes to infinity, energy
  does too, but probability of having photon of
  high energy goes to zero

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Photo Electric Effect
Photons (particles of light) knock electrons out
atoms like billiard balls
Photon
Electron
The Bluer the light, more K.E.
The more intense the light, nothing happens to
K.E.
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More particle-like behavior of light
Compton Scattering
If a photon scatters off of an electron at an
angle q, what happens to the energy of the photon
and electron?
e-
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Compton Scattering
e-
-Without doing any algebra, we can argue the
outgoing photon must loose energy since some is
imparted to the electron (energy conservation)
-Since both Einstein and Planck tell us Ehf,
then the outgoing photon must get redder
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Compton Scattering
e-
We wont do the algebra, but here is where you
would start
Energy Conservation
See why we call it Quantum Mechanics!!!
Conservation of Momentum
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Compton Scattering
Result
Books Notation
Sure enough, the scattered photons wavelength is
larger (frequency smaller), and so the photon
appears redder
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Particle or Wave?
Planck, Einstein and Compton showed that light
can behave like a particle (photon) contrary to
the picture given by electromagnetism.
But, light does act like a wave too diffraction,
refraction, and polarization are all phenomena
only consistent with wave theory.
Recall Einstein came up with special relativity
because the new theory of electromagnetism gave
the speed of light always to be c.
Electromagnetism always was consistent with
relativity.
However, relativity naturally leads to the
concept of the photon- a massless particle that
travels at the speed of light.
Hmmm.
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Principle of Complimentarity
Niels Bohr said quit being so thick, its both
particle and wave!
Particles and waves are abstractions of the
human mind. When we try to conceive of what light
really is, we insist on a visual picture. Yet
there is no reason why light should conform to
these modelsThe true nature of light- if that
means anything- is not possible to visualize.
-Giancoli
Whoa thats deep. For physicists anyway.
Told you this was good bed-time reading!.
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Getting Weirder
O.k. so light is both particle and wave, but how
about particles with mass- they should just be
described as only particle-like, right?
Nope they can be described by waves too! De
Broglie Wave
It starts from using relations for photons
since
For photons, since there is no mass
Now we just make a big logical leap and say that
this applies to matter!
Equating
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But it does
Electrons being diffracted by a crystal, just how
waves would be diffracted by a grating
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Names______________________________________
PH103, de Broglie Waves
1. Calculate the de Broglie wavelength for the
following a) A 142 gram baseball thrown at 90
mph (40 m/s). Recall h6.626x10-34 Js b) An
electron is given a kinetic energy of 10 keV
(since KE is smaller than the rest mass, use the
non-relativistic equation for kinetic energy).
Note h4.135x10-15 eV s and hc1240 eVnm,
me0.511 MeV/c2 Note Trick that can make
calc. easier c) Looking at the equation for de
Broglie wavelength and the previous examples,
when is the wavelength (and therefore wave-nature
of matter) noticeable? 2. For many electrons
like those in 1 b), At what distance from the
central max would you find the third diffraction
maximum after they are sent through a double
slit. Here the slit spacing is 0.05 nm apart and
the distance to the screen is 10 m?