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Physics 222

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Title: Physics 222


1
Physics 222
  • D.S. Durfee

2
(No Transcript)
3
The Physics Revolution of the 20th Century
  • Relativity physics of the fast
  • Quantum Mechanics law of the small
  • Atoms clocks, discharge lamps, lasers
  • Molecules chemistry
  • Nuclei fission, fusion
  • Solid State semiconductors
  • Degenerate Matter superconductors
  • Coherence Superposition quantum computing

4
Puzzles at the Beginning of the Twentieth Century
  • Null result of the Michelson-Morley Experiment
  • Ultraviolet Catastrophe
  • Photoelectric Effect
  • Maxwells Equations Spell the Demise of Atoms!
  • Discrete atomic emission lines

5
Blackbody Radiation
  • Maybe better called Thermal Radiation
  • Sun, hot burner plate, incandescent lights

6
Cosmic Microwave Background
from http//aether.lbl.gov/www/projects/cobe/CMB_i
ntensity.gif
7
COBE Map of Microwave Background
from http//aether.lbl.gov/www/projects/cobe/COBE_
Home/cmb_fluctuations_big.gif
8
Blackbody Radiation
Ultraviolet Catastrophe
9
Max Planck, Plancks constant
  • Found an empirical formula which fit the measured
    blackbody curve
  • Looked for way to derive this empirical formula

Max Karl Ernst Ludwig Planck
Photo from http//www-groups.dcs.st-and.ac.uk/his
tory/PictDisplay/Planck.html
10
Plancks Law
11
Max Planck, Plancks constant
so... Light is a Particle!
wait a minute... Light is a Wave!
Max Karl Ernst Ludwig Planck
Photo from http//www-groups.dcs.st-and.ac.uk/his
tory/PictDisplay/Planck.html
12
Are you pondering what Im pondering?
  • Why do we only have problems with the short
    wavelength (high frequency) part of the blackbody
    spectrum?
  • Is the energy in a light mode quantized because
    light can only exist that way, or because the
    atoms that emit it can only emit quantized
    amounts of light?

13
Hints of Quantum Mechanics
  • Quantization of Matter
  • Very early idea, not really proven until 1900
  • Nature of mater not understood until much later
    (Rutherford, etc.)
  • Quantization of Electric Charge
  • Faradays law of electrolysis (1833)
  • Zeeman, q/m
  • J.J. Thompson discovers the electron
  • Millikans oil drop

14
Strange or Common Sense?
  • Does it bother you that mass is quantized?
  • Does it bother you that charge is quantized?
  • What are some differences in the way that mass
    and charge are quantized?

15
Pay Attention...
You need to know this to do the homework!
16
Statistics...
It represents the electro-magnetic energy in a
box per unit volume per unit frequency...
How much electro-magnetic energy is there inside
my oven?
17
Quick Writing Assignment
  • In one minute, write a short, clear, and concise
    paragraph which explains how Max Planck came to
    the conclusion that light was quantized.

18
What did Einstein Win his Nobel Prize for?
photo from http//www.th.physik.uni-frankfurt.de/
jr/gif/phys/einst_7.jpg
19
Puzzles at the Beginning of the Twentieth Century
  • Null result of the Michelson-Morley Experiment
  • Ultraviolet Catastrophe
  • Photoelectric Effect
  • Maxwells Equations Spell the Demise of Atoms!
  • Discrete atomic emission lines

20
The Photoelectric Effect
21
The Photoelectric Effect
22
Photoelectric Effect Intuition
  • What would a plot of current vs. light intensity
    look like?
  • What would a plot of current vs. voltage look
    like?
  • What would a plot of threshold voltage vs. light
    frequency look like?

23
Wave Theory of Light and the Photoelectric Effect
  • Existence of a threshold voltage
  • Current increases with intensity
  • Threshold is a function of light frequency
  • No intensity threshold
  • No time delay
  • Fairly monochromatic electron energies

?
?
?
?
?
?
24
Einsteins Explanation...
Planck was right!
photo from http//www.th.physik.uni-frankfurt.de/
jr/gif/phys/einst_7.jpg
25
What is the velocity of the electrons coming off
the metal?
26
Quick Writing Assignment
  • In one minute, write a short, clear, and concise
    paragraph which explains why the existence of a
    threshold voltage in the photoelectric effect
    suggests that light is quantized.

27
Hmmm
  • Does Plancks work on blackbody radiation prove
    that light is quantized?
  • Does the photoelectric effect prove that light is
    quantized?
  • But dont we already have proof that light is a
    wave?

28
Double Slit Interference
29
Single Photon Interference
30
X Rays
  • Discovered in 1895 by Roentgen
  • Generated when electrons hit target
  • Materials transparent to them
  • Not deflected by magnetic field
  • Roentgen saw no interference and little
    diffraction!
  • Bragg diffraction explained in 1912
  • X rays are EM waves with wavelengths from 0.01 to
    0.10 nm

31
X-Ray Tube
32
Bragg Diffraction
?in?out
33
Bragg Diffraction
2d sin(?) m ?
34
Bragg Diffraction
35
(No Transcript)
36
What do X-Rays Have to Do with Quantum Mechanics?
37
Compton Effect
  • Compton noticed that x-rays were softer after
    they had passed through a substance.
  • Compton showed that this could be explained by
    treating scattering as collision between a
    photon of light and a particle of matter

38
Compton Scattering of a Photon
Before
After
e-
e-
Are you pondering what Im pondering?
Yes, but how will we ever get that many rubber
bands?
No you buffoon! Why do you need x-rays to see
this?
39
Quick Writing Assignment
  • In one minute, write a short, clear, and concise
    paragraph which explains why the Compton effect
    suggests that light is quantized.

40
Puzzles at the Beginning of the Twentieth Century
  • Null result of the Michelson-Morley Experiment
  • Ultraviolet Catastrophe
  • Photoelectric Effect
  • Maxwells Equations Spell the Demise of Atoms!
  • Discrete atomic emission lines

41
Radiating Atoms
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