Physics 222
D.S. Durfee
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
Puzzles at the Beginning of the
Twentieth Century


Null result of the Michelson-Morley Experiment
Ultraviolet Catastrophe
 Photoelectric Effect
 Maxwell’s Equations Spell the Demise of Atoms!
 Discrete atomic emission lines

Blackbody Radiation

Maybe better called “Thermal Radiation”

Sun, hot burner plate, incandescent lights
P  eAT 4
u( )d  Energy radiated per unit frequency
i.e. the amount of light radiated which has
a frequency between  and   dv.
Cosmic Microwave Background
from http://aether.lbl.gov/www/projects/cobe/CMB_intensity.gif
COBE Map of Microwave
Background
from http://aether.lbl.gov/www/projects/cobe/COBE_Home/cmb_fluctuations_big.gif
Blackbody Radiation
Max Planck, Planck’s 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/~history/PictDisplay/Planck.html
Planck’s Law
8X
u ( )  X / kT
e
1
5
Max Planck, Planck’s constant
E  n  fh
Max Karl Ernst Ludwig Planck
Photo from http://www-groups.dcs.st-and.ac.uk/~history/PictDisplay/Planck.html
Are you pondering what I’m
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?
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
–
–
–
–
Faraday’s law of electrolysis (1833)
Zeeman, q/m
J.J. Thompson discovers the electron
Millikan’s oil drop
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?
Pay Attention...
You need to know this to do the homework!
Statistics...
What does u( )d really represent?
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?
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.
What did Einstein Win his Nobel
Prize for?
photo from http://www.th.physik.uni-frankfurt.de/~jr/gif/phys/einst_7.jpg
Puzzles at the Beginning of the
Twentieth Century

Null result of the Michelson-Morley Experiment

Ultraviolet Catastrophe

Photoelectric Effect
 Maxwell’s Equations Spell the Demise of Atoms!
 Discrete atomic emission lines

The Photoelectric Effect
The Photoelectric Effect
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?
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

Einstein’s Explanation...
Planck was right!
photo from http://www.th.physik.uni-frankfurt.de/~jr/gif/phys/einst_7.jpg
What is the velocity of the
electrons coming off the metal?
1 2
mv  h  
2
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.
Hmmm…

Does Planck’s work on blackbody radiation
prove that light is quantized?

Does the photoelectric effect prove that
light is quantized?

But don’t we already have proof that light
is a wave?
Double Slit Interference
Single Photon Interference
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
X-Ray Tube
+
Tungsten
Target
Anode
-
Filament
Cathode
Bragg Diffraction
θin=θout
θ
d
Bragg Diffraction
2d sin(θ) = m λ
θ
d
Bragg Diffraction
What do X-Rays Have to Do with
Quantum Mechanics?
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
Compton Scattering of a “Photon”
Before
After
ee-
Are you pondering what
I’m pondering?
Yes, but how will we ever get that
many rubber bands?
No you buffoon! Why do you
need x-rays to see this?
Quick Writing Assignment

In one minute, write a short, clear, and
concise paragraph which explains why the
Compton effect suggests that light is
quantized.
Puzzles at the Beginning of the
Twentieth Century

Null result of the Michelson-Morley Experiment

Ultraviolet Catastrophe

Photoelectric Effect

Maxwell’s Equations Spell the Demise of Atoms!
 Discrete atomic emission lines

Radiating Atoms